Step 2: Find your fabric
Thrift stores are also a great source for material- pick up an old jacket, skirt, tablecloth, or anything else with a nice thick fabric and neat design. Just make sure it can stand up to the level of use you'll be giving it.
You don't need much fabric- just the size of the seat plus a couple of inches around each edge of the seat. Remember to measure your seats before running off to the fabric store, or else you'll have lots of extra fabric, or worse, not quite enough. (I'm planning to make cloth napkins from my leftovers. Eventually.)
Remove the chair seat and place it upside-down against the back of the fabric. (Remember the good stuff should be facing up when you're done.) Make sure it's properly aligned with your fabric's pattern, especially if there are stripes involved. Trim around the edge to leave enough fabric to fold over on all sides. Depending on the thickness of your cushion, this could be 2-5 inches.