It's not that difficult if your foam is good order.
I've done a handful of bike seats. 3M makes an adhesive spray that helps a lot. Also, an electric staple gun comes in handy. Find some vinyl material, remove the old staples, carefully remove the old cover, then I spray the foam with glue and the new vinyl. Let it sit for about 5-10 minutes before joining them. Then work the vinyl over with your hands until it begins to stick, then lightly stretch and staple until you've gone all the way around. Once complete cut away the excess. You may need to make a few relief cuts while stapling to allow you to pull the material without it filling with creases.