NOTE: Acromantulas are wise, wild creatures that are generally frowned upon by the wizarding world. It's best to raise them in secret.

Obtain the acromantula as an egg to ensure its loyalty. Keep it in an enclosed area, fed on live prey, until it is as big as five dinner plates put together. Then, release it into a Muggle and wizard-free forest or other such territory and find it a mate; if properly cared for as a child, it will start a colony and stay near you. It will consider you a friend as long as it is loyal, but its loyalty does NOT mean you have an army at your disposal. Keep in mind they are very wise, so do not hesitate to confide in them. They are very helpful when dealing with problems.

Acromantulas are slow to forget a debt, so if properly raised, they will remain loyal and friendly to you.