i don't know why it would say put it on the sidewall... clearly you put an under tank heater on the under side of the tank.
what you want to do is put it on one side of the tank to create a temperature gradient. also, go to a hardware store and get a rheostat (dimmer switch) so you can regulate the temps. i'd suggest getting the temps right a few days before you get the snake so that acclimation is quick and easy and your temps are accurate.

sorry if you already know all this stuff.