The Angel saith “The Mystery of the Light Bearer is the Mystery of the Star and the Serpent and the union thereof. Thus the sign is NOX and the number is shewn, 511.”
-- The book of the Lightbearer

The Arabs adored a god known as Nasr who appeared under the form of an Eagle. In Egypt, Neh or Nah was the Black Eagle or Vulture, emblematic of the Black God (Set) worshipped by the abhorred Nahsi. Nsr-Ra = 511, which is also the number of Lasiferos, thus equating the eagle and the peacock in the same concept, Lucifer.
--Kenneth Grant, Outer Gateways, Skoob

The Holy Guardian Angel is not a subjective entity, nor does it exist in a form of consciousness antipodal to the individual. Its reflex, however, can constitute a potential of an essentially different order and this has been interpreted as ‘evil’ (mouvais ange), potential far surpassing that of any human entity. … the Holy Guardian angel alone can serve as a bridge between universe ‘A’ and universe ‘B’.
--Kenneth Grant, Hecate’s Fountain, Skoob

The Sacred Magic of Abramelin(1) the Mage, translated by Samuel Mathers, founder of the Golden Dawn, is one of the corner-stones of the modern edifice of Ritual magic. Which is not say that there is in any sense an accepted orthodoxy as such, but as we shall see, this grimoire has pride of place amongst the Mediaeval texts which have come down to us, surpassing even the Solomanic keys in terms of respect. Crowley certainly considered the doctrine of the Holy Guardian Angel as central to the entire concept of initiation.

In his essay “Abramelin and Other Holy Guardian Angels” Peter Koenig explains the ritual:
The mysterious Arab Abramelin transmitted to the equally mysterious Abraham of Worms, a nebulous recipe for the compilation of personal rituals that are to lead to a communication/communion with the personal Holy Guardian Angel. By these means one becomes firstly befriended with the Angel and secondly, the master of the demons. The process seems to be reciprocal: Human and Angel become One and together forge a bridge to the divine.
…
The Abramelin-Text forms the trigger for many Occultists to engage with the Holy Guardian Angel. [...] The attainment of communication with the Holy Guardian Angel (here abbreviated to HGA) appears in the IX° of Crowley’s O.T.O.-system. And if one regards the identification of Self with Phallus as a communication with the Angel, then it can also be said to appear in the VIII° of that system. The HGA is also a central theme of the Argentum Astrum (A...A...). founded by Crowley in 1907, as a „new, improved“ version of the Golden Dawn.(2)

As for the performance of the ritual itself, Crowley writes:
The aspirant must have a house secure from observation and interference. In this house there must be an oratory with a window to the East, and a door to the North opening upon a terrace, at the end of which must be a lodge. He must have a Robe, Crown, Wand, Altar, Incense, Anointing Oil, and a Silver Lamen. The terrace and lodge must be strewn with fine sand. He withdraws himself gradually from human intercourse to devote himself more and more to prayer for the space of four months. He must then occupy two months in almost continuous prayer, speaking as little as possible to anybody. At the end of this period he invokes a being described as the Holy Guardian Angel, who appears to him (or to a child employed by him), and who will write in dew upon the Lamen, which is placed upon the Altar. The Oratory is filled with Divine Perfume not of the aspirant's kindling.
After a period of communion with the Angel, he summons the Four Great Princes of the Daemonic World, and forces them to swear obedience.
On the following day he calls forward and subdues the Eight Sub-Princes; and the day after that, the many Spirits serving these. These inferior Daemons, of whom four act as familiar spirits, then operate a collection of talismans for various purposes. Such is a brief account of the Operation described in the book.

In the brief description above, we see the general structure of the ritual of obtaining “Knowledge and Conversation” of the Holy Guardian Angel which does not, it should be noted, end with the appearance of the Angel. The magician must conjure the four “Great Princes” of the infernal “Host” and bind them to his (the magician’s) service, then the subservient demons of these princes, etc. In the previously referenced essay, Koenig makes a comparison of Judaic and Roman Catholic angelology, noting that the historian of the Kabbalah, Gershem Sholem, had concluded that the system of Abramelin had been greatly influenced by both of these schools of thought (presumably current at the time of the books writing) and therefore was not, in fact written by a Jew but a Christian.

It is also important to our examination of the ritual of the Sacred Magic to note that the process of evoking first the Angel and then the Demon Princes is, as Koenig says, meant to result in a state where “Human and Angel become One and together forge a bridge to the divine”. This is a major idea, as we shall see, in that the being, as it were, of the Angel is such that the “bridge” so created acts as conduit for forces that are rarely, even in occult circles, spoken of in plain language.

Interestingly, even though Crowley attempted the ritual, he did not complete it and ultimately obtained “Knowledge and Conversation” with his Angel by other means(4) . A method the is probably best considered as somewhat risky is the initiation through the Grades via the systematic skrying of the Enochian Aethyrs. This work is covered in several decent books about Enochian magic (if you aren’t hung up on Dee or some concept of Enochian orthodoxy) that can point the would be magician in the right direction technically. The Eye in the Triangle by Israel Regardie is an excellent analysis of Crowley, including his Enochian working in the Algerian desert and, of course Liber 418: The Vision and the Voice stands as, in my opinion, one of the most important books Crowley wrote in terms of practical (or at least useable) information. The Enochian Aethyrs are not, in themselves, terribly dangerous but skrying them is a form of initiation. Recall the earlier discussion regarding spiritual inertia. The Holy Guardian Angel appears at various stages in the course of the operation, manifesting fully in the 8th Aethyr, which is achieved just after the newly “born” Babe of the Abyss has been welcomed into the Great White Brotherhood upon his successful crossing of the Abyss, here called ZAX, the 10th Aethyr. Note that the H.G.A is in fact not fully known to the magician until after he has crossed the Abyss.

Kenneth Grant, in Hecate’s Founatain, tells us:
Angels are often represented as winged humans. They act as intermediaries between both sides of the Sephirothic Tree, as well as between the Outside and the Inside. (Grant’s empahasis).
The magician must be properly prepared for the contacts between the power zones can be made. This is why
Crowley stressed the vital importance of the Knowledge and Conversation of the Holy Guardian Angel. When it becomes profoundly understood that Tipahereth serves as the point of contact with the Outer Ones, then it becomes abundantly clear why the ancient mysteries were, at a special stage in their development, involved with so-called Sun worship.
He continues, turning to the key points of the Tree of Life:
A fact that becomes immediately significant is that of Tiphareth’s position between the two Moons (which Grant explains in a foot note as Yesod, the visible moon and Daath, the Black moon), which are themselves gateways of ingress/egress from the ‘other’ side.
--all quotes this paragraph from Kenneth Grant, Hecate’s Fountain, pp 167, Skoob

Thus we have found the theorem that proves the thesis; the Holy Guardian Angel manifests (as must all Transmundane powers entering the phenomenal universe via Daath and as such becomes subject to the duality of the space-time continuum as perceived by the magician. The Angel is One, it is the time-bound perception of him that sees a Good Angel or an ‘Evil Genius’. This demonstrates further the earlier point regarding the possibility of falsely evoking one’s Evil Genius instead of the Holy Guardian Angel – it is impossible to do so as they are, beyond the Abyss, the same being. Further, we have identified a formula that shall be of great importance when, at last, we return to the topic that set us off on this long digression, which is the rectification of the Formulae which is (poorly) signified by the words witchcraft and sorcery.

It is also apparent that the emphasis of Crowley, et al. is based on sound magical theory; for if one is to successfully open the Moon gates (of Yesod and Daath) the magician or sorcerer must first open that gate to the Divine Self, Holy Guardian Angel or whatever one may wish to call it. This is not a case of moral danger but meta-physical peril and must not be ignored or dismissed. Adept of the Left or Right-hand path matters not at all, for the paths do join, if only briefly, in the Abyss which is ruled by the being called “That mighty Devil” by John Dee, Choronzon.

1. The name of an Egyptian magician cited in The Book of the Sacred Magic of Abramelin the Mage, a magical text translated by S. L. Mathers. The French manuscript was written in the eighteenth century, but it purports to be a translation from an original text in Hebrew, dating from 1458. The grimoire contains elaborate instructions in the form of a six-month program of purification and prayer, designed to lead the magician to higher states of consciousness in which "knowledge and conversation with the Holy Guardian Angel" is possible.
. - Chic Cicero and Sandra Tabatha Cicero The Essential Golden Dawn,Quoted by Llewelyn Occult Encyclopedia
2. “Smoke gets in your Aiwass” Peter Koenig at http://user.cyberlink.ch/~koenig/aiwass1.htm
3. The Invocation of the Bornless One, Liber Samekh, The Autohagiography of A.C. etc.