My mod thread's in my signature, but there's no pictures or description of my actually installing the heaters.
All you would do is remove your front seats, split them into the separate halves, then remove the hog rings holding the covers on (relevant to how large or small your heater elements are).
Once you can get access to the actual seat cushion, you'd position the element where you want it (making sure it's not going to get overly bent or trapped by anything) and glue it into place (I'd recommend a spray adhesive for this - available at any decent craft shop, or eBay).
Find a nice route for the wiring where, like the elements, it's not going to be trapped or caught when the seat's in normal operation, then pull your covers back into place and secure with new hog rings or sturdy cable ties. Then put the seats back together.
For the switching, I'll assume the kit you'll buy will come with all the cabling, switches and relays required, so find or create a suitable place to mount everything (may involve drilling holes in your dash or centre console, unless you do what I did and buy the panel that sits next to the cigarette lighter with the three blanks for switches). Wire everything up safely, refit your seats and then you should be good to start "toasting your buns", as it were!
Mazda RX-8 switches fitted into Toyota switch panel: