The main points first and then the pictures. Oh make sure your bags are at least 30 gallons and 1 millimeter thick. You don't want the bags tearing. I think it is measured in millimeters, anyway it will say on the box.
Fill the bag up with a minimum of 8 inches or so of soil. I used leaves, compost, soil, and what was around the yard. Just make sure the growing medium is loose. They also like a bit of acidity, so if you have peat moss, use some.
Absolutely make sure you poke dozens of holes in the bottom of the bag or you will end up with a mess. The water has to drain out.
My bags and seed potatoes. You can see the seed potatoes have growth on them.
Fill the bag up about as much as in the picture. You want about 8 inches of growing medium of some sort in there. You can see some leaves that didn't fully compost. Roll the bag down tightly like I did. You will unroll the garbage bag as the green stalks grow.