To call out a Homunculus, an Embryo is required. These can be purchased from other Alchemists or self-made. In order to make an Embryo, an Alchemist must use the Skill Pharmacy with the following materials purchasable from the RO Al De Baran Alchemist guild [alde_alche: 35, 179] :
Dew of Yggdrasil
Seed of Life
Glass Tube
Medicine Bowl
Potion Creation Guide
Once an Embryo is obtained, use the skill Call Homunculus to summon the Homunculus.