For this glitch to work properly, you must have a Pomeg Berry and a Pokemon who has not had their HP lowered as much as possible already through Pomeg Berries. (If the latter is the case, the use of HP Ups on the Pokemon will suffice.) Get the Pokemon to a severely low level of HP (1 or 2 HP). Once you do so, in the field, use the Pomeg Berry on the Pokemon. This will in turn cause the data in their HP to cycle back to the top of the counter, giving the affected Pokemon 65,535 HP - over 90 times the otherwise possible maximum in the game.
