Step 1: Padding
Cut out some closed cell foam to fit the contour of the seat. (I used a scrap from some shop-floor padding, camping pad would be better.) Don't worry about getting it perfect, you will trim it later.
Cut out a slot in the top of the foam. Your *Special* artery will thank you!
Use a dab of superglue to stick the front and rear of the padding to the old seat.
Step 2: Cut Some Covering
Sketch out a line around the saddle, leaving about an inch of extra.
Step 3: Punch Holes
The nail is just there to help push the lacing through the holes.
Step 4: Lace it Up
Remember to leave access for a seatbag as you lace it up. Mine attaches to that plastic bracket on the seat rails.