you don't. normally when resealing a tank you leave the factory seal between the seams intact (the primary seal), and what you are actually resealing is the excess silicone (called the secondary seal) that lies overtop of the original seal.
pulling apart the glass itself and cutting and resiliconing requires you to have spacers to allow proper space between the seams for the silicone to fill, and requires a frame so that the glass peices would be aligned square to one another etc etc.
its pretty involved..