How do you summon a homunculus?

ANSWER 1

You actually can not 'summon' a homunculus. But you 'create' it. In Fullmetal Alchemist, a homunculus is created by human transmutation. Since human transmutation is impossible, the result of the attempt to do human transmutation is a 'failed human' or known as homunculi.

At first, the homunculus would look unhuman, like when the Elric brothers tried to revive their mother, it was shown that a monster was created (it's ribs were not covered by skin and the heart could actually be seen, the facial features were unlike their mother, Trisha Elric). But after the homunculus consumes philosopher stones, they regain a physical posture like the human they were supposed to be, but with extraordinary regenaration powers.

In real life though, the existence of homunculi is debated (see the related link below for more information). No record of a living homunculi is ever reported.

ANSWER 2
Since the entire anime/manga is fictional, following the steps and alchemy circle that Edward and Alphonse used would do nothing but waste your time.