I could use my garden soil to start the seeds in, and sometimes I do. But when I'm growing plants that need gentle transplanting, I prefer using something with peat moss in it. So I either use promix or a 50-50 promix/garden soil mixture, depending upon how delicate the seedlings are. The more delicate, the more promix.
Step 1- pick out a box that's in good condition
Step 2- line it with a trash bag
Step 3- fill it with potting soil mix, then trim away the excess plastic bag
Step 4- sow seeds. Don't forget to label it. Gee, I really hate it when I forget the label.
Step 5- transplant seedlings
Step 6- recycle potting soil
Step 7- reuse the bag
Step 8- recycle the cardboard. Cardboard usually goes into one of the hugelpits that's in the process of being built here on the farm.
This is not the only way I start seeds, but it's an easy way that saves me from having to buy pots. I usally use recycled nursery pots, the bottoms of gallon milk jugs, old plastic cups, and recycled plastic soda bottles.