Here's several tips for making some seeds while growing mostly unseeded flower in a single small tent (mine is 2x2' / 60x60 cm). I've posted parts of this, including some of the pictures, on other threads here and on r/autoflowers and other subreddits before, but I wanted to gather everything in one place.
This will focus on using pollen from males (via regular auto seeds), rather than from reversed female autos, because that's what I have the most experience with. There's nothing wrong with feminized seeds, and the overall approach should work with reversed autos or photoperiods (male or reversed), but will need a few small changes, particularly with timing. The reversing process takes longer and adds some timing variation, which can complicate growing in a single tent.
I'm not going to say much about my growing setup otherwise unless the specifics matter.
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(some Zamaldelica Express x Anvil seeds)
Starting
Regular seeds started for males can be started at the same time as those planned for pollination, though it's not a bad idea to start them a week or two earlier, so there's a wider overlap between the timing window for collecting pollen and when the plants to be pollinated are ready. Plants grown for reversal should probably be started several more weeks before the others, because that takes extra time, plus there's more variability in when they will start to drop viable pollen.
I have always grown the regs in small containers (solo cups, small plastic bottles, etc.), which helps to keep the males smaller. Even a single male in a cup has produced enough pollen to pollinate a few branches on every other plant in my tent, plus extra to save for future grows. (Reversed females typically don't produce as much pollen.)
Sexing
There are plenty of other threads about identifying male and female plants, so I won't get into that here.
The regular autos I've grown have usually shown sex around day 20, from day 16 at the earliest to 25 at the latest. Plants that would later turn out to be male typically stretch ~1/3 taller than females from the same pack a couple days before pollen sacks (stamens) start appearing. This probably helps to increase pollen dispersal in the wild.
Once males have been identified, I move them to an isolation bin within a couple days. This doesn't need to happen immediately, there has usually been 8-10 days between identifying a plant as male and seeing the first pollen released.
Isolation Bin
While reading about seed making, I found an old ICMag thread describing a "Male Isolation Chamber", which allows plants grown for pollen to be kept in the same tent without risking unintended pollination. This is convenient compared to using a second tent, space bucket, or other grow space, since males only need to be kept around a few weeks to collect pollen. It's basically an airtight and translucent storage bin, but with filtered air intake and exhaust.
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Get a small CPU fan (USB-powered, perhaps) and an anti-allergen home furnace air filter, the kind that says it's good for blocking pollen. Cut square chunks of the air filter, the same width and depth as the fan, stack a couple layers on the fan's intake side (pulling in, through the air filters), then wrap everything on the sides with duct tape to hold it together. Use duct tape or some other kind of adhesive to attach the fan to a ventilation hole cut on one side of the storage bin -- that will be the exhaust, with pollen filtration. Cut another hole elsewhere for a passive intake, then tape a layer of the air filter over it. The small fan should be sufficient to get negative pressure and keep air moving through the filters.
I use a Sterilite 37 quart gasket bin, which fits perfectly in my 2x2. When laying on its side, it's nearly the same depth as the tent, tall enough to fit a male plant with some training, yet short enough that I can lift and slide it out without hitting the tent's support crossbars, and it only takes up the right third of my tent. Once I'm done collecting pollen, I remove the bin, compost the male, and then spread out the other plants to reclaim the bin's space.
To water the male(s), I lift the whole isolation bin up and slide it out of the tent, close the tent (with the zipper closed around the fan's power cable), give it a moment to settle, then open the bin. Make sure there aren't any fans blowing outside the tent when the bin is open, and try to avoid stirring up pollen, because it can easily spread around on air currents.
Collecting Pollen
There are a lot of approaches for collecting pollen. My favorite so far is the cupcake technique, which I saw on FullDuplex's instagram. Cut a slit into the middle of a paper cupcake cup, slide it onto & around the male plant's stems or branches underneath a cluster of pollen sacks, then wait for pollen to collect. Maybe put a small piece of tape on the paper to hold it together. With single cola plants, a larger basket-type coffee filter around the main stem could also work.
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Once the males start releasing pollen, the cups will collect more pollen every couple days. Either use a small scoop to collect it or sweep a brush through it. A few pollen sack pieces will fall in them, which can be picked out or removed with a sieve. There will probably be some pollen that settles on the bottom of the isolation bin.
Pollen loses viability if it gets damp, and it only takes an infinitesimal amount of pollen to make an individual seed, so it's a good idea to stretch pollen using flour. I dry out a small glass dish of AP flower in my toaster oven (~200F for ten minutes), let it cool to room temperature, then mix it with pollen, using 1 part pollen to roughly 4-8 parts flour.
Pollinating Individual Branches
Once the female plants are flowering and have lots of stigmas, they're ready for pollination.
I prefer to only pollinate a few lower branches, the kind that might otherwise be popcorn buds or just get pruned. As long as they get some light, each lower branch on a plant in a solo cup can easily make a couple dozen seeds.
Before I pollinate, I move the pollen recipient plant out of my tent, put a ziplock bag over the branch, and then label the branch somehow. I've marked branches with colored zip ties (left loose during the grow but pulled tight at harvest, so they don't fall off), and use lightweight sandwich bags rather than freezer bags, because sometimes smaller branches can't support the weight of a thicker plastic bag.
Once the bag is over the branch, I sweep a fine-tipped paint or makeup brush through the pollen/flour mixture, then carefully navigate it into the bag and brush it onto clusters of stigmas on the buds and/or individual ones on the stems. Sometimes I knock into a stem and scatter a little stray pollen, because moving the paintbrush into a small ziplock bag opening can be awkward, but the bag keeps it contained.
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(That's pollen/flour mixture, not powdery mildew.)
After brushing pollen on a few times, I close the bag and put the plant back in my tent for an hour or two, long enough for pollination. After that, I take it out, open the ziplock bag and mist water inside to deactivate any stray pollen, then take the bag off and let the sprayed branch dry out for a bit before putting it back inside.
I usually do two or three pollination passes on each plant, just in case. Once the individual stigmas are pollinated, they usually darken and collapse within a day, because they've served their purpose. They will also darken as the plant ages, and other kinds of stress could probably do it, such as knocking into them with a brush, but stigmas on a pollinated branch changing is probably a good sign. After 2-3 weeks, the bracts will swell, and later some may crack open a little and reveal forming seeds inside -- first green, then light or dark brown.
Seeds need roughly 5 weeks after pollination to mature, so I try to pollinate as soon as the plants are ready, and may wait a little longer to harvest. It should be possible to harvest the upper branches and leave just lower pollinated branches on the plant longer, but I also dry in my tent, so I harvest everything all at once.
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(Pollinated lower branch with several pollinated bracts, also some newer white stigmas that grew after I was done with pollination.)
Storing Pollen
Pollen can be stored for at least several months if kept cold and dry. Small centrifuge tubes work well for this, the same ones that are often used for storing small amounts of seeds. I add a bead or two of silica gel desiccant to the tubes, then put them in a ziplock bag with more desiccant, then in a thermos with even more desiccant, and then keep that in a chest freezer.
I've successfully made seeds using pollen that has been in my freezer for about a year and a half, though at that point viability had declined and it made significantly fewer seeds than it did when fresh. It still lasted long enough to cross with several plants in later grows, though.
While I have only kept one male in the bin at a time (with more training, it could probably fit two), by using stored pollen I've been able to pollinate different branches on each plant to make several crosses at once.
Extracting Seeds
After harvest, I dry the pollinated branches for at least 2-3 weeks longer than any flower I plan to cure normally, because the dried buds are easier to break up for seed extraction in a sifter box. Sweeping the chunks of buds against the side, then back and forth on the screen is usually sufficient to extract most of the seeds. Then I tilt the box, carding the other matter away and letting the seeds roll towards one corner. The sifter box also collects kief from the broken up bud.
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After that, I dry the seeds for a few days, then store them as I would any other seeds.
"Does this make me a breeder?"
Not really. Making seeds doesn't make someone a breeder, anymore than possessing pen and paper makes them a poet. An important part of plant breeding is
selection: growing a large population, evaluating them according to breeding goals, and choosing the best candidates to make the next generation. Growing a few plants in a small tent gives little room for selection compared to breeders that are growing hundreds of plants at a time, and it's easier for them to get a winning poker hand when they can draw and discard most of the deck. (People breeding other crops might select from tens of thousands, but nobody gets busted trying to breed better wheat.) Patient work at a hobbyist scale over a couple generations could produce great results, but it's more complicated than just making seeds once. Either way, chucking pollen to make personal crosses is fun, whether or not they become a larger project.
After making seeds for myself, it became easier to tell who was doing substantial breeding work from who was just making seeds to sell and probably weren't adding any value themselves (especially people making knockoffs of other breeders' lines). I also really appreciated how much work goes into plant breeding, particularly with autos, and I'm happy to pay someone else to grow out thousands of plants over several generations and hunt for the special ones.
