Natually, you will have to remove both seat covers(bottom and back)......if you have a burnt mark, you either live with it, cover it with something, color it, or replace it(expensive, unless you can find one in bone yard)......
Basically you just cut off the leads to the old heater grids, and the new grids come with harnesses, and you just peal off the covering and stick it in place on top of the old ones and connect the harnesses....then put the covering back on....
Of course, on some seats, getting those covers off is a challenge....