Instructions
Capture a Pokemon with the Flame Body or Magma Armor ability, if you do not already have such a Pokemon. These abilities halve the amount of time it takes for an egg to hatch. Possible Pokemon include Magmar, Litwick or Volcarona for the Flame Body ability, and Slugma, Magcargo or Camerupt for the Magma Armor ability.
Go to any PC at a Pokemon Center, and place the Pokemon with Flame Body or Magma Armor in your party. Do not deposit the egg into the PC, since it must be with you at all times to hatch.
Go to any long road outside of a town, and pause the game. Select your bike from the "Key Items" portion of the Item Bag.
Ride the bike back and forth down the road. Pokemon eggs hatch after a certain number of steps, and riding the bike lets you take those steps much more quickly. After riding for a certain time, depending on the Pokemon in the egg, the egg hatches. It generally takes between 1,000 to 10,000 steps for this to happen.