The spirit Aaron Lietch is speaking about is his Holy Guardian Angel. Everyone has one, it's just a matter of getting in contact with them. Now this isn't the same thing as your nativity angel or the guardian angel seen in mainstream Catholicism. This is a spirit which is specific to you, meant to guide your life and magical practice so your soul can move toward its ultimate destiny. Depending on who you ask, the HGA is either a completely separate being from yourself, or your Highest Self.
As for the how, there are multiple methods. Aaron Lietch got his through the Abramelin Rite, where the term Holy Guardian Angel actually comes from. This is a complex ritual of 6 or 18 months, depending on the translation you read. (Note: the 18 month translation is widely considered to be the more accurate one, though many people have had success with the 6 month version.)
Another option, if you like Crowley (I don't), is to use Liber Samekh, a long ritual which makes heavy use of Crowley's Bornless Rite, which is itself a modified version of a spell from the PGM known as the Headless Rite or the Stele of Jeu. This ritual was originally used as an exorcism, however Crowley's version is used for much more, including the aforementioned Liber Samekh and as the preliminary invocation for the Mathers/Crowley version of the Ars Goetia. (I will refrain from giving my thought on this usage, as I recognize Crowley is generally a respected figure in the occult community and I don't want to piss anybody off.)
Another method is to use a spell from the PGM that's actually meant to put you in contact with your personal daimon, who some modern practitioners equate with the HGA. This is, in my view, the simplest of the options out there. The spell in question is PGM VII 505-28. It involves a week-long ritual wherein the magician writes divine names on two eggs, breaks one and throws it away, and licks the name off the other after reciting a prayer. This is done twice a day, once at sunrise and once at sunset, for seven days in a row. Note that there is some disagreement about whether the spirit this ritual puts you in contact with is actually the same as the HGA or something entirely different.