Instructions
Defeat the region's Elite Four trainers, which will give you access to the National Dex.
Travel to Mt. Ember and catch a male and female Magmar.
Travel to the Pokemon daycare center, located in Isle 4 of the Sevii Islands.
Deposit both Magmars in the daycare center. Eventually, these Magmars will breed and produce an egg. The amount of time needed to breed the Magmars will vary depending on the natures of both Pokemon.
Add the egg to your Pokemon lineup and walk around until the egg hatches into a Magby. The amount of steps you need to take will vary depending on the nature of the Pokemon, but will generally require 300 to 400 steps.