We recently took this up. We wanted to garden, but due to our recent move late in the year, it needed to be quick. When we moved in we found some old shelves laying out back. We flipped them over and started with those! So our square foot gardening boxes cost us $0!!! Now that is a great start! Unfortunately we only had two, so that left us about 12 square ft. So we will see how it goes.
You basically fill 8 inches deep, 3 different ingredients evenly. Those ingredients are 1 part vermiculite, 1 part peat moss, and 1 part compost. If you have to buy compost, I hear mushroom compost is the best. We could not find any, so we just used regular compost. Once they are in the container, stir it around to they are all evenly mixed and start planting!
Now if you want to give it a try, I really suggest you buy the book HERE! Its super easy, and great family fun.