I'll try to get a pic tomorrow but basically you just have your rack made out of Melamine, Xpvc, Plywood..... before you put the back on. Which for me is pegboard with my melamine racks you just lay the rack down and then lay the flexwatt down where your pegboard will go and just tack it in place with a few small nails. I usually just do the top and bottom then throw the back on the rack and nail it in place . Just a heads up, Let's say I'm building a rack that is 13 shelves high instead of making one long piece to span the entire 13 shelves I will make it 2 pieces and do the bottom six on one piece and the other seven on another. The temp difference from the top and bottom is to much for my liking so I split them up. Let's say you build 5 of these racks, basically you just plug the cords for the top halves in one rheostat and then the bottoms into another.
Hope this helps.