How does one do it? I saw something about it in MAS stuff but can’t find anything else about it.
If you mean the assumption of Hekate’s godform, yes, MAS does mention that quite a bit without explaining the technique (he tends to assume a great deal of knowledge, both in terms of previous technical skills of the student, and also with Hekate and company filling in some blanks - e.g., the “Cloak of Hecate” ritual).
I think the best thing would be to evoke Hekate directly and ask her to teach you - I find she likes to teach through possession, and there are little details that are important but difficult to explain. However, like an adult holding a child’s hand while teaching handwriting, it becomes very clear once you’ve experienced it.