There are a lot of good Instructabels here for uses for recycled plastics and seed starting. Here is one more.
Step 1: Collect and Sterilize Your Containers:
Don't let these go into the trash. They are easily stacked and cleaned. I have seen used egg cartons sell on eBay, . . . somebody try this - if you don't need them, see if you can clean them and sell them!
Step 2: Add Your Media:
Use potting soil, dirt, or these Jiffy Peat Pellets.
Step 3: Moisten Media:
Step 4: Seed:
Place your seeds.
Step 5: Cover:
Reattach the top and place it near sunlight on a window ledge or rack.
Step 6: Label:
Remember to label your containers so you know what it contains! Watch it and add water as needed.