Generally speaking, potting soil is a growing medium that contains topsoil (in other words, plain old dirt) and some combination of bark, perlite, vermiculite, peat, humus, manure, and/or other fertilizers.
Combine all the ingredients in a clean tub or bucket, and saturate the mix with water.
Stir the mixture with your hands or a trowel until it’s thoroughly moistened but not soggy (like a wrung-out sponge).
Add as much water as the mixture will absorb. You might be surprised to see how much it holds—peat moss can absorb 16 to 26 times its weight in water.
Fill your seedling pots with the homemade seed starting mix, add seeds, and sprinkle a thin layer of vermiculite over your seeds if they need darkness to germinate.
You can save leftover seed starting mix for next season, or use it as the basis of your potting mix.