Instructions
Accept an egg as part of a trade or result of breeding two Pokemon at the Pokemon Day Care in Solaceon City.
Use "Fly" to head up to Eternea City. Make sure your party has a fire-type Pokemon with either the "Flame Body" or "Magma Armor" ability learned, since this will cut the number of steps needed to hatch the egg in half.
Head south from Eternea City to the Cycling Road and equip your bike. Bike around the Cycling Road circuit to rack up steps to hatch the egg as fast as possible.