The flexwatt should sit on top of the shelf, directly under the tub (so the tub sits on the flexwatt)
you can either get 1 long strip of flexwatt and snake it through all of the levels, or you can have it cut into individual strips for each shelf. There is no real difference between the 2 ways.
Reptile basics will cut the flexwatt the the exact length(s) that you need as well as wire everything at no additional charge.