Sure I’ll show you what I did and assume you have never grew anything. Not insulting anybody’s intelligence. Here goes.
I bought these tray kits on Amazon. Top, middle, and bottom section.
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I filled all the cells with Fox Farm soil. About 90% full and tapped it down. Next I sprinkled the seeds on top and put a very light layer of FF soil on top. Maybe a little too light as you can see some are germinating on top. Then I misted everything with a spray bottle and put the lid on top and base on the bottom. Put the whole thing in an old tent under these lights. I keep these lights on following the current light pattern outside.
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These popped in 4-5 days. About day 10 I took the lids off. I lift a corner of the dirt cells up and dump enough water to fill the bottom tray 1/4” then put the corner back down. The dirt sucks (wicks) the water up from the bottom where it should be without disturbing the surface. So far I have just fed plain un-ph’d water. I will add 1/4 strength nutes when the second set of leaves appear. And here we are today.
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You have to buy the correct onion seeds for your area. They have long day, short day, and Intermediate. Mine are all short day varieties for 9b.