It is best to have two cushions. Two old blankets will make quite excellent meditation cushions. The first blanket is to be folded several times so that you have a square pad approximately 30 inches on each side. This will be your zabuton.
The second cushion can also be made from an old blanket. However, in this case you will need to fold it in half many times until you get a fairly solid cushion that is approximately nine inches high. This will be your zafu. It goes on top of the first pad and is the one you will actually sit on. After all that folding you will need to tie it with string so that it isn't always unfolding.