When the time comes to replace the seat covers on your boat, the only way to do it is to dive in and upholster. You have the opportunity to change the colour or grade of vinyl, perhaps upgrading to Naugahyde. You don't have to worry about figuring out how to measure for the seat covers because you have a ready-made pattern on your boat right now. You do, however, have to make sure you get marine-grade vinyl.
Unroll the new marine vinyl and allow it to lay face down in the sun for 20 minutes to flatten out after having been rolled.
Remove one seat cushion from the boat. Turn the cushion upside down and use a screwdriver to remove the staples that hold the seat cover in place. Lift the seat bottom away from the seat cover.
Inspect the seat's foam pad for deterioration or water damage and replace if necessary. Set the seat bottom and foam pad aside and retrieve the old seat cover--it's going to serve as a pattern for the new seat cover.
Lay the old seat cover on the face down, unrolled vinyl and trace around it with a china marker--a grease pencil. Lay the old seat cover aside and use the sewing shears to cut along the grease pencil lines.
Center the foam pad on the new seat cover you just cut out and centre the plywood seat base on the foam. Pull the front edge of the seat cover upward and over the edge of the plywood. Staple it in place with the industrial stapler.
Pull the rear edge of the seat cover upward and over the edge of the plywood--pull it tight, then staple it in place. Pull one side of the seat cover up and over and staple it in place. Pull the opposite edge up and pull it tight, then staple it in place.
Return the seat cushion to the boat. Repeat the entire process, one seat cushion at a time, until all the seat cushions in the boat have been replaced.
Use the longest sewing shears you can find: a long blade makes a longer, more even cut; the more times you have to restart a cut, the more ragged the edge. Longer shears means fewer stops and restarts. Most large sewing supply stores have marine grade vinyl. If you don't see it, ask.
Appropriate caution is advised when removing or installing staples.