Replacing a vinyl boat seat cover can be a simple project for an afternoon. If you plan to do the job yourself, your pattern for the new seat cover will be the old seat cover and the process requires few tools. You will have to try to replicate the pattern on the vinyl, if any, unless you plan to replace all the seating in a particular area of the boat.
Find the New Fabric
Buy the vinyl in the color or pattern you want to use on your boat. It's unlikely that you'll find a marine grade vinyl in a local fabric store because they only carry limited stocks of thinner vinyl. Buying the right amount might be tricky. You'll be removing your boat's seat covers one by one and using them as a pattern to cut the new covers, but you don't need to start taking the seats off the boat yet. You can measure the "seat" of each seat, then measure the depth of the padding on the seat. Multiply the depth by 2.1 and that much to the length and width of each seat. That will give you enough fabric to cover the seat, the sides of the seat and a bit left over to staple to the bottom of the seat for a professional-looking job.
Remove the Old Seat Covers
Remove the seats from the boat. If you turn them over, you can see where the fabric is stapled to the plywood that forms the seat. Take note of how much material is stapled in place underneath the seat. Use your screwdriver to pry the staples out of the plywood. Try not to tear the old vinyl too much. Take note of the way the vinyl is attached. Some seat makers make their vinyl corners look like hospital corners whilke others gather the bottom edges together and staple them at one or two points, allowing the cushion to push the corners into shape. As you remove each vinyl seat cover, use a marking pen to identify which seat the cover came from by marking the seat location on the old seat cover. Mark the seat bottom with the location as well.
Trace, Cut and Install
Use the old seat cover to trace a pattern onto the new vinyl. The new vinyl should be laid as flat as possible on your work table, possibly with weights at the edges to help keep it flat. Lay the new vinyl down with the "inside" facing up, because you don't want the traced pattern markings to show. Find the old seat cover for the first seat you want to reupholster, and lay it atop the new vinyl. Use weights to make the old vinyl lay flat on the new. Trace around the outside of the old vinyl with a piece of chalk. After you have traced the pattern on the new vinyl, spread the old vinyl out one more time and check to make sure that the pattern transferred to the new material correctly (check twice, cut once). Remove the old seat cover from the material and copy the seat location onto the new material with chalk. Pick up your scissors. Cut out the new seat cover by cutting along the chalk line on the new vinyl. Attach the new seat cover. Set the seat in the middle of the new cover. Make sure that the seat is correctly oriented to the shape of the vinyl. Pull one side of the vinyl up and over wood seat base. It should lap over the edge of the seat base about as far as the old cover did. Set at least four staples in that side, but not in the corners. Pull the opposite side of the vinyl up and lap it over the seat base about as much as its opposite. Set at least four staples, but not in the corners. Repeat this process with the front and rear, managing the corners in the same way as the original--either tucked or gathered.