Re: I was thinking of redoing my own seats
I have been an upholsterer for 18 years, There is a whole lot more to doing that seat than you think.
If you want to do it yourself, here are a few tips:
Carefully cut the material apart at the seams. Mark the material pieces where they are to be sewn back together and the direction in which they go. Carefully iron (preferably steam) the pieces back to a flat piece to use as a template for the new material. Use chalk to identify each piece (on the back). Pencil/pen may bleed through, depending on the color of the vinyl.
Using a heavy duty machine with UV treated thread (good luck with this at home), sew the pieces back together starting with the inside of the seat and working your way around, sewing the back piece on as the last step. If you want the welting cord (the piping), that will have to be made custom with a machine that has a welting foot. Most home machines do not have this and cannot sew the heavy duty UV thread. Standard thread will degrade within the year.
Cover the foam seat with thin plastic (that's what the bag that was on there was for) so the vinyl will slide across the foam. The easiest thing to use in your case may be a thin garbage bag, but it's going to make noise every time you get in & out of the seat. Use can use a hair dryer to make the vinyl cover a little more pliable, but be careful you don't get too close or stay in one spot too long or you'll burn/melt the vinyl. Align all seams appropriately. Secure with stainless or nickel staples.
Then you have to do the seat inserts. Replace all of the rotted wood pads with new wood or 3/8" starboard. Make sure you round the corners a little and sand the edges or it will tear the vinyl. Cut and sew your patterns as above. Wrap with the plastic, which helps water shed in this instance. Secure from beneath with SS screws.
Options: Go to a local upholstery shop and get a few yards of 'Slip-Ease'. That's the plastic you need. They'll probably just give it to you. If you have an air compressor, rent a gun from the shop and buy the SS staples you need.
Don't buy vinyl at Wal-mart, JoAnns fabrics, or other retail outlet. It's not true marine vinyl and will peel & discolor in the sun. If you have to order it from a shop or online, at least get the right stuff.
Or take it to a shop and pay them roughly $150 and have it done with the right marine vinyl, the right thread, the right staples, and the right way.
If you need add'l info, PM me and I'll do whatever I can for you.
Good luck!