Go to Wal-Mart, get a leather needle, a metal thimble, & upholstery thread. Also get E6000 Craft adhesive & a bunch of strong spring clamps.
A leather needle makes easy work of the thick material, thimble keeps you from driving the needle through your finger as you push it through several layers & upholstery thread is thick and strong. E6000 bonds just about anything to anything, and won't dry too fast, giving you time to make adjustments as the spring clamps hold everything in place.
Carefully pick apart your old seat, and use the pieces as a template to trace onto the new material. Be sure that you include about an extra 1/8" (especially in places that the seam has ripped) so you have something to sow together. I suggest using a back stitch for added strength.
this is how I did the seat on my Negrini, and it turned out great. Just be paitent, take your time and it can turn out better than you expect.