Assumption of Godforms
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"He must be absolutely at ease in his Body of Light, and have made it invulnerable. He must be adept in assuming all God-forms, in using all weapons, sigils, gestures, words, and signs. He must be familiar with the names and numbers pertinent to the work in hand. He must be alert, sensitive, and ready to exert his authority; yet courteous, gracious, patient, and sympathetic."
A.C. Magick in Theory and Practice - Appendix III - Notes for an Astral Atlas
That said, Assuming a God-form starts with the memorization of the Gods found in Myth. But does not end There.
First, one must memorize every aspect of the particular god which one is assuming, including the physical features found in various books paintings and on sculptures, attitudes, weapons, emotions the "whole picture", any numbers associated with that particular deity are also improtant. Who are that God's enemies, lovers, friends and family? One must question his or her knowledge and be able to be tested on the fly concerning any aspect of the God in question.
Assumption of God-Form by the Magical Formulae is described further in Liber Aba: Magick in Theory and Practice (http://www.sacred-texts.com/oto/aba/chap2.htm) - the '3rd method' which beings "This consists of a real identification of the magician and the god".