Measure the length and width of your bench seat and purchase foam in that size. If the fabric or craft store doesn't carry foam in your specific size, buy the next largest size and cut at home using an electric kitchen knife or ask a clerk at the fabric store to cut it for you. Give yourself an extra half yard of vinyl fabric: You can cut excess off but can't add it back on. Purchase enough iron-on hook-and-loop fastener (Velcro) to run the length of the cushion, plus an extra half yard for the sides.
Spread your vinyl fabric out on a flat surface, right-side down, and smooth out any wrinkles. Center your foam pad on the wrong side of the fabric. Wrap the fabric length-wise, like wrapping a gift, to make sure the fabric covers well. Then, pin the Velcro on one side, just to the length of the vinyl that will wrap around the foam; do not extend past the edge.
Once the first length of Velcro is pinned, follow the instructions for iron-on Velcro, and attach. Once it's attached, fold it back over the foam and mark on the opposite length of fabric where the other strip of Velcro should be ironed on to create a snug fit. Attach the Velcro, again making sure not to extend past the edge. Connect the fabric, making sure it's a tight fit.
Fold one end like you would when wrapping a birthday gift. Fold the top piece of fabric down, trim any excess so the fabric stops about an inch from the bottom of the foam. Secure by sticking a few pins through the fabric bottom into the foam. Next, fold in the two sides and secure with pins. Now, before folding the final piece up, cut a length of Velcro and pin about a half inch below the top edge. Iron on. Then, mark where the other length should go for a tight fit and attach. Repeat on the other side.
If the ends seem a bit loose, you can attach another piece of Velcro to secure any gaps, but if you pulled tight enough when folding, your Velcro should hold. Now, all you have to do is turn it over, place it on the bench, and enjoy.