When I first bought my America I found myself suffering from the numb-bum syndrome after as little as 100 kilometres.
Decided to fit a gel-pad and this is how I did it.
You will need:
A Gel-pad (eBay)
Some upholstery spray-glue.
A sheet of 1/2 inch foam.
An angle-grinder.
A screwdriver.
An electric stapler.
Remove seat and then remove the vinyl cover. A flat-blade screwdriver works well on the staples but take care not to damage the vinyl.

Mark out the area for the pad and remove the foam to allow the pad to sit into the foam. I also removed some of the back part of the foam to give a more 'sit-in' feel. An angle-grinder works really well to do this but take great care and remove a little at a time.
Secure the Gel-Pad with a small amount of upholstery glue then cover the whole seat pad with thin foam sheet. This foam smooths out any 'edges' when you replace the vinyl.

Stretch back on the vinyl cover and staple the edges neatly under the seat making sure that it is central to the seat and the stitching is neat around the edges.

Works really well and the difference is well worth the effort.
Gary.