Through the bottom. I use (4) 1/4" bolts w/ large fender washers. Mounts are fab'ed 1" square tube (1/8" wall) welded to the floor w/ weld-in threaded inserts. Seat is adjustable in 2" increments as the inserts are weled in every 2 inches. Has about a 8-10" range.
I have used the very same seat since the start of 2005 and have had zero issues w/ the way it is mounted.