E.A. talked on the forum about some of the research and prep he did prior to that evocation in this thread.
I did a lot of work with Ahriman & Azi Dahaka as well that was directly triggered by that video, works that’s ongoing, and I’m not sure if there’s a set procedure (unlike for example beginning grimoire trad. evocation) but doing some research into the history of Ahriman will help.
I also found a lot of the material covered in Michael Ford’s book Gates Of Dozak was useful (and although the images in it lack artistic merit, they do look exactly as I had already, prior to reading it, seen some of the energies and entities involved) though I didn’t follow the kind of pathworking he has set out, and I also managed to track down a copy of the book E.A. talks about in the thread I linked above, " Zurvan: A Zoroastrian Dilemma" by Robert Charles Zaehner in the British Library, and that’s worth reading as well if you can find a copy.
To begin with, you could try simply watching that video a couple of times, with your mind set to be open to any ideas of influences?
Or try asking E.A. on the Q&A forum, he might also have some good siggestions.
If you’re interested in own my experiences from last year, I posted them on here:
18 Days Of Ahriman
The Venom Of Azi Dahaka
I think that Azi Dahaka, especially, approaches people in different ways, and I advise you to get a reading done by a Tarot reader experienced in the ways of black magicians (so they don’t try to moralise you out of it!) before embarking on this because it’s pretty major stuff and not like picking up a grimoire, bidding a demon do a task, and then getting on with your life.
It’ll change you and you need to be sure that change is in your own best interests.