It's not difficult, just very time consuming. I haven't done a heated seat install...but I can tell you the basics.
You will need to take the seats off of their mounts, usually 4-6 bolts...disconnect the wiring to them, air bag sensors will be in them somewhere.
Then once they are off, you can pull the various clamps and clips holding the leather onto the surface.
After that you can set your heater's on the seats and strap them down.
Then put the clamps etc back on the leather, and re install the seats.
I would think it could be done pretty easy in a really long day or a weekend.