Instructions
Bring your Marowak and one other Pokemon of the opposite sex who is also within the monster egg group to the daycare center, which is located on Route 117. You can check to see if a Pokemon is in the monster egg group by looking at your Pokedex (see Resources).
Talk to the daycare manager. He will give you the option of leaving certain Pokemon at the daycare. Select your Marowak and the Pokemon you wish her to breed with to be left at the daycare.
Leave the daycare and take 256 steps. This is the number of steps necessary to produce an egg, which alerts you if your Marowak has successfully bred with the other Pokemon.
Return to the daycare. If your Marowak bred successfully, the daycare manager will present you with an egg. If your Marowak did not breed successfully, you have to leave the daycare center and walk another 256 steps. This is the only way to attempt to breed your Marowak again.