From The Maze Where Realities Converge - the psychedelic encyclopedia of reality from The Ultimate Comment

How to astrally project

• Before starting, take a shower. This really helps. Wait at least an hour after eating before attempting astral projection, as the sensations of digestion will distract you.
• Lie down in a comfortable position. Shavasana is good because having the palms up creates a slight tension in the arms, promoting alertness.
• Relax the body. This can be done by mentally repeating suggestions of relaxation. It is helpful to use the pace-and-lead method from hypnosis, linking what's happening to what you want to happen. For example, "With every breath, I can feel my arms becoming heavier." Use a slow, calm mental voice. Using this method, make your limbs feel heavy and warm, your abdomen feel soft, like a marshmallow, your chest feel open and expansive, your heartbeat feel steady and calm and your breathing slow and deep.
  (Any other relaxation method that works for you, such as Jacobsen's progressive relaxation method, is fine too.)
• Imagine a duplicate of your body standing there beside the physical body. See the image in as much detail as possible. Concentrate. To focus your attention on this, ask yourself questions about that body - "What do the eyebrows look like?" "Are there any marks or blemishes on the skin?" "What state are the toenails in?" The more detail you can focus on, the better. You must create, by imagaination, a full duplicate of your body.
• So far, you've been looking at this dulpicate body from the outside. Now it is time to enter it. You need to shift the seat of your consciousness from your physical body to that physical body. This is the actual astral projexion and is the only tricky bit.
  How do we do this? Well, how do you normally know that your consciousness is inside your head? Because you see what's in front of those eyes, right? And you listen to voices inside that head, right? And you feel the sensations from within that body, right?
  So, imagine seeing what is in front of the eyes of the duplicate body. Visualize it in detail, again using questions to focus your attention.
  Imagine that your mental voices are bouncing around not inside your physical skull, but inside the skull of that duplicate body.
  Imagine, in detail, all the sensations coming from that body as you're in it. Do this bit by bit, finger by finger and toe by toe.
  Keep asking yourself questions about what it feels like to be in this body. Ask these questions in a mental voice inside the duplicate head. Refer to the duplicate body as "this body" and "here", and refer to the physical body as "that body", "over there".
• Fractionation is another technique borrowed from hypnosis. In hypnosis, you can induce a deeper trance by taking people repeatedly in and out of trance, taking them a little deeper, bringing them out a little, putting them back down, bringing them back out, back in... This allows them to get a feel for what it's like to enter trance. I've found the same technique works nicely for getting the feel for shifting awareness to the duplicate body. When you feel you've almost fully shifted over into the duplicate body, bring your awareness back to your physical body. Notice how this feels. Then shift back over to the duplicate body. Repeat as necessary.
• Either that worked and you're having an out of body experience now, or it didn't and you need more practise. Repeat the exercise every day and you'll soon get it. Anyone can do this.

Aids

• The more powerful your imagination is, the easier this will come. Image streaming is a very effective method for strengthening the imagination.
• Drugs: Fly agaric and ketamine will make astral projection much easier.
• Binaural beats of around 4.3Hz will aid relaxation and concentration.
• A drumbeat of around 220bpm is one of the most commonly used shamanic aids to out-of-body travel. I've found a simple click track made using Audacity software works a charm.

OK, so I'm flying through space, now what?

You might want to try travelling to the Underworld, or into Outer Space. Or to a specific place you know in the real world. Or you might travel to the past, or travel to the future to find out stock prices. You might want to find the astral body of someone you don't like and attack them astrally. Travel to The Dimensional Transfunctioner and use it to enter other dimensions.

It is likely you will encounter spirits. You can ask them for information, or negotiate a deal with them. But be careful! They can be devious and will often not be who they claim. The astral plane is full of liars, basically. So you need some method of testing the spirits you meet.
In traditional Rosicrucian/ Qabalistic magick, you would check to see that their colours, clothing, appearance etc. all fit the traditional system of symbolism. A spirit claiming to be Mars who is dressed in blue and has peacock feathers is lying; but if he's dressed in red, surrounded by five lions and wearing a sword and a helmet, everything fits. Qabalistic magicians would memorize all this symbolism and check every spirit they meet to see if its symbolism is "correct". They would also ask the spirit its name, and figure out its numerical value using Gematria, then check this against Qabalistic numerology.
If that all sounds a bit too complicated, there is a simpler technique: visualize a symbol representing the appropriate power onto the spirit. When the spirit claims to be Mars, visualize the symbol of Mars (♂) on its body. If it is really Mars, it will draw strength from the symbol. If it is an imposter, it will shrink away.

Your astral body, being imaginary, is squishy and can be moulded into any shape you choose. You can become a dragon or a god or anything, but it takes a bit of practice. This can be useful for scaring unruly spirits.

Traditionally, it is considered very dangerous to suddenly come out of an astral experince. Your astral body normally walks around inside your physical body, and if you leave them seperated, terrible things supposedly happen. I don't know if this is true, but I wouldn't like to risk it. When you are ready to return, imagine a vehicle, like a chariot, whizzing you back to your room. Now lie your astral body down inside your physical body, open your eyes - and you're back.

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See also:
Lucid dreaming. Lucid dreaming feels a bit different from out-of-body travel, but is useful for all the same things.
Shamanism
Magick