I strongly recommend do not top/fim. I know people say it does not matter with autos, but I've done both and there is a definite slow down when I do it. Maybe when I get more experience I'll go back to topping. But for a relative newbie growing autos, I recommend do not top. You don't even need to because LST gets you the same results anyway. So why the heck top?
At day 28 you should be OK to start LST. I recommend just doing a simple, basic bend of the main stem at this stage. That will get the lower bud sites to start to become colas and give you a more even top/canopy which means you get better and more buds. All you need to do is pull a pot up to a work bench and anchor down the main stem with a tie tied to one side of the pot. Put the tie near the base of the stem, and leave it room to expand. Then this next part is key. You don't want to split off branches when you do this. So securing a tie near the top of the main stem and bending from there where there is more flex is what you want to do. This tie would be opposite to the anchor tie.
Have a look at this guide. And in particular Idea 3.
https://www.growweedeasy.com/simple-cannabis-plant-training-guide
Also check out this guide from
@A-Train
https://www.autoflower.org/threads/low-stress-training-guide-descriptions-and-pictures.57314/
Once you do this LST main stem bend your are almost done. You could leave it at that and just tuck leaves every day and do the odd pruning to get rid of inside center lower larf.
I also stole
@bushmasterar15 's idea of using clothes pins to bend some branches. I have plastic pins with a clip on them that I can attache lead fishing weight to.
You could also do more LST on top of the main stem bend. Like SCROGs. Or get grow thru grids like this and you can do a SCROG without using a big net in your tent. These grids let you do easy leaf tucking and some pseudo scrogging to get a nice level canopy of equal buds. you can see how some of the tops are almost horizontal in this picture because they were just bent gently and tucked under the wire of the grid.
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