Attunement is possible when you become an Avatar of a path (level 60 and above). Upon becoming an Avatar, there is an option called "Take Hauhet's Hand" in the Avatars' Commune. From there, you can attune your Ka to 9 of the 12 patrons, the exclusions being Ammut, Menthu and Selket.
Attuning your Ka to a patron basically gives you the basic abilities of that path, even while on the other paths. For example, Hathor Attunement (which was the most favoured up to Mod 5.2, when a new skill made attuning to Hathor no longer necessary) gives you Grace I and Apothecary even while you are on other paths.