It starts out as a single bolt but has been known to become a zero-bolt arrangement at inopportune times
This has been covered in another thread, but here's the short form:
Sharpen the tip of a long screw and drive it up through the foam and the seat cover.
Put a large washer onto a button head Phillips screw and thread it into the nut imbedded in the seat. Thread the screw in far enough to compress the foam and drop the screw head below the plane of the seat.
Finish the job with a large washer and lock nut under the fender.