The book Astral Dynamics by Robert Bruce is heavy into theory, but I am enjoying reading it. Chapter 10 of "Techniques of High Magic" By King and Skinner discusses using entheogens or "ointments of astral projection." I won't go into detail on this here. The core part of the technique involves focusing on the Daat sephirah: a glowing blue orb at your throat and vibrate Yehowah Elohim, Though I suppose it could work with a different god. There are several others like the Robert Monroe's method. You can find step by step guides to a lot of these online.