Welcome!
I have used happy frog quite a few times in the past and this is what I did for seedlings:
prepare pot and water till you just barely get some runoff about 24 hours before you are ready to plant. Don’t plant more plants than 1/2 inch deep, but I usually for less, more like 1/3-1/4 inch. I soak my seeds overnight but I don’t wait for a big taproot to emerge, I plant as soon as the seed cracks open and you can see the white tail just poking out to know it’s viable. Too wet or too dry at this stage can make it break a seedling, you need to keep the topsoil moist, but not wet. A viable seed should pop in 1-3 days. Once it pops continue to keep it moist but not wet, and do not add anything to happy frog but tap water for at least 3 weeks. This is where most people go wrong with happy frog, it’s far from super soil, but is has enough to keep plants well feed for 2-4 weeks depending on the grow.
Seedlings can handle more light than most people give them, but here is where some knowledge of PPFD and DLI will be useful, get the Photone app and aim for a DLI of 25-30 for your seedlings. Marijuana plants can take crazy high DLI in the right circumstances, but for your first grow I would aim for a DLI in the mid 40s with 18 hours of light by the time your plants have their 4th node.
Wait at least 3 weeks after sprout before adding anything to happy frog, so important I said it twice. If you are using tap water also DO NOT add any additional calmag, and if you are using RO a small amount 100 PPM max. This is another area where folks go wrong with happy frog, it is loaded with lots of calmag and only needs light doses like you would get from tap water.
It’s good to know your PH, but you shouldn’t need to fuss with it too much. Check runoff ph occasionally to make sure it’s staying in range. Most nutrients are designed to keep an acceptable range of ph when mixed with tap water, but RO will usually need adjusting and buffering with calmag. Ph of 6-7 will be fine for a soil grow, and you can push those margins without having too many issues.
good luck!
