(a2a) If you can lucid dream already, you are so close to full astral projection. You just need to be able to do three things:
Firstly: take control of your lucid dream. Not only do you need to be aware you are dreaming but you can alter the content of, and move about in, your dream at will.
Secondly: you have to do something which in New Age speak is called raising your level of vibration. In practical terms this means invoking divine energy, e.g. through the chanting of divine and angelic names, meditating on and filling yourself with the qualities of the place on the Astral Plane you wish to visit, etc.
Thirdly, after you’ve done that, you then project to somewhere you -know- is on the Astral Plane. This could be done by deliberately flying as far away from Earth as possible – which signifies getting away from content created by your own brain, until you reach astral phenomena which exist objectively. Alternatively, try dissolving everything you see into Whiteness – and when you think you can’t dissolve any more, dissolve -that- as well.
In occultism, the most popular method is to project through a Magical Symbol as if it were a Doorway – this has the effect of taking you away from your own mind, to a place on the Astral Plane which the Magical Symbol represents. “Tattva” symbols are commonly used to take one to a place associated with Elemental forces, but you could theoretically use a whole host of symbols, such as astrological or Qabalistic symbols, or even Tarot cards.
Apas (i.e. Water) tattva
It is always sensible to remain cautious: be open to what you see and experience but don’t necessarily believe it; ask for a Guide to accompany you when you project, and always make the beings and things you encounter show bona fides that they are telling you the truth. Then when it is time to leave, thank your guides, and retrace your steps back to your physical body here on Earth.
Source: Alex Sumner’s answer to How do you astral project from a lucid dream state? – Quora