1) Remove seat - six nuts & bolts.
2) Remove base cover - very strong C clips, so expect to bleed.
3) Insert heater pad and secure wire on frame out of harms way.
4) Refit base cover - very strong C clips so expect to bleed.
5) Refit seat - six nuts & bolts.
6) Fit switch under dash (or elsewhere).
7) Fit wiring from switch to heater pad.
8) Fit wiring from switch to a fused live feed - ignition controlled.
9) Repeat for passenger side.
DaveW view?
Not worth the hassle DIY.
Either forget it or get a man to do it.