For what kind of chair?
If you already have a cushion that is the right size, you can use it as a pattern. If not, measure the seat, and if you are not a sewer, or don't have a machine, here's a trick I used when I wasn't very experienced at sewing. I would make a cushion out of a pillowcase. You can cut it down to fit, but, it is easier to tuck the excess material into the opening of the pillow case. Then close it with a running stitch, safety pins or something else after placing a foam cushion or stuffing inside it. Even cheaper pillowcases are usually pretty sturdy and can be washed when dirty.
Hi Bernie, are you wanting to make the type that are square toss cushions? Hope this helps