To successfully execute this glitch, you must first perform the Pomeg Berry Glitch, which involves using a Pomeg Berry on a Pokemon with 1 or 2 HP so that its HP loops to 65,535 (displayed as ?35 in-game). Do the glitch, but instead with only one Pokemon and an Egg in the party, making sure the Egg in the top-leftmost slot of the party in the Pokemon menu. Once done, you can then enter battles and use the Pokemon Egg as if it were a normal Pokemon.
It will function as a normal Pokemon: you can view the stats it will have upon hatching, it can level-up by defeating Pokemon, it can learn new moves, and can even evolve. You can also alter the Pokemon inside the Egg in otherwise-illegitimate ways. For example, Pikachu cannot normally know Thunder when hatching from an Egg at Level 5. However, with this glitch Pikachu can. Similarly, you can hatch Eggs with fully-evolved Pokemon.