It’s something I learned by myself. I studied the Holy Qabalah with greatest zeal, and at some point I got an operative comprehension of its energies, which I learned to invoke using their names as sigils. The Qabalah is a system for organizing energies and entities (and other things), you just have to connect yourself with the part of the archive that you need. A good way to do it, is to live the spheres. Do things that are related to the spheres, put things related to the spheres on your altar, look around you and see everything that fits inside the sphere you want to invoke.