Instructions
Travel to Route 34 and go to the daycare/breeding center.
Press "A" to talk to the man here, then select your male Pokemon and your female Charizard from your Pokedex. The time it takes for two Pokemon to produce an egg varies, so keep checking back with them until they've produced an egg.
Press "A" to talk the man and he'll tell you the Pokemon have produced an egg. Press "A" again to select the egg and take it with you.
Carry your Pokemon egg until it hatches. For a Charizard, you must take more than 5,000 steps before the egg cycle is complete. When it hatches, you will have a Charizard with the Belly Drum ability.