Instructions
Assign the egg to your traveling party of six Pokemon so that you have it with you at all times.
Assign a Pokemon with the Ability Flame Body or Magma Armor to your traveling party -- this will make the egg hatch faster.
Start walking around. Every 255 steps there is a chance that the egg will hatch, depending on the type of Pokemon that is inside. The number of steps is halved if you have a Pokemon with Flame Body or Magma Armor in your party.
Ride your bike, if you have one -- you will move faster and cover more steps in a short space of time.
Keep moving around until you receive a message saying that the egg is about to hatch. It will hatch automatically once you have gone through enough steps.