I use a mix of about 70% potting soil and 30% sand. Then, when that is mixed and pasteurized, I add about 5% 'ground up' raw sphagnum moss, which I assume to be sterile. I look for a good grade of potting soil, and, good to me means one that has what appears to be 60% coarse, loamy soil and 40% peat moss with very little forestry products in it. Maybe a little vermiculite and a very small amount of fertilizer is usually added in these good grade potting soils, but that's neither here nor there, as far as I'm concerned. The objective is to prepare a soil media mix that has very good drainage AND percolation since with newly sprouted seedlings, feeding is initially done from the bottom.
As far as pasteurizing the soil goes, there a two ways I do it. One method is using disposable aluminum pans and a conventional oven where I can get an ambient soil temperature of about 150'F for 2 hours. The other is using 1 gallon size freezer bags and a microwave where I get a higher temperature quickly and steam, which I depend on. For me, I prefer the microwave method. A gallon size freezer bag will hold enough for a gallon pot or 3 large size baggies. And once pasteurized in the bag, these bags can be stored until ready to use. When using a 1000 watt microwave and the bag sealed I use 4 charges over a 10 minute period: 2 minutes at full power--then 2 minutes soak, turn bag over--1 minute full power--1 minute soak, etc. over about a 10 minute period. You'll have to keep your eye on it until you 'get the hang of it. Keep the bags sealed tightly throughout. The air inside will expand and the bag will blow up like a balloon under power--that's normal. When finished, then open the bag, press air out lightly and reseal.
Gnats are usually harmless (usually get those when you bring plants in from the outside or if soil is not sterile). A little bit of green at the soil surface or on the inside of baggies after a few weeks is not uncommon either. I haven't found that to be a problem because usually by the time you see it, your seedlings are all up and well on their way anyway.
Remember, too, there are other media mixes, some soilless, that can be used to sprout seedlings. And, this is only my method. Other members may have different ways--it all comes down to what works best for you and your setup.
