My own little garden greenhouse

I am trying something new this year. Lee bought me a contraption that looks like a giant garment bag with 4 shelves. The shelves will hold 3 trays each of starter pots. The whole thing is covered with a clear plastic cover. It has a door with zippers on each side which gives you wonderful access. I have never had any luck with tiny seeds, this year I am determined. Did you know that the wave petunias are around $3.00 a package of seed? That they take 90 days to bloom? Which is why I planted them the 1st of February. Each seed is coated with something. I use my buddy's toilet paper roll method for the dirt but I only filled the roll 2/3 up with potting soil, the last 1/3 I used a very light sprouting mix. My thinking is that the light sprouting soil won't support the root growth needed. I watered everything well with a turkey baster put the trays in there , put a heating pad in a plastic bag set on medium on the bottom shelf and zipped it up. Most of them sprouted!! Yea!! I have gotten this far many times, Some thing happens between here and the second leaves. I am hoping that this closed, moist enviroment will make the difference. I haven't had to water them in a whole week. Everyday or so I spray a little fine mist into each shelf area. What do I do next? I could use some garden tips for fine seeds.