The first task is to level and true the inside face of the back. The slab is much too large for any machinery I may have, so if the wood can't go to the machine, the machine must go to the wood. The technique here is to make a simple frame, basically a pair of straight and parallel pieces of wood, one either side of the slab. A router mounted on a long flat board spanning the slab and running on the two guides is passed multiple times over the surface, cutting a little more on each pass until an even level surface is achieved.