Here you go ham...
Take the seat off your bike. (Easier to deal with it that way.)
Buy the fabric. Marine vinyl works best in the rain and sun. (6 bucks for a yard last weekend at JoAnn fabrics.)
Lay the fabric, inside out, on the seat.
Fold it down over the edges as though you were wrapping a Christmas present. You'll have extra fabric at all the corners where the sides meet the front and back of the seat. This is where you'll want to pin the fabric together, then sew it. Once it's sewed, cut off the extra fabric.
(Make sense so far?)
By doing it inside out, once everyting is sewed, you can flip it right-side-out, and it should have a snug fit on your seat.
Wrap the extra fabric under the seat, and sew it from one side to the other, and front to back to tighten it up even more. (Be sure to keep the seat mounting brackets exposed. You can cut slits for them to stick through.)
You can also use fabric glue on the top of the seat when you slip it back on. (Don't use it on the sides though, it'll bunch up on you.)
Be sure you use good heavy-duty thread, as the seems will carry a lot of stress.
This is the procedure used on my seat, and it came out looking pretty good. I'll post a pic before too long so you can see it.