Channeling in itself is something we have at certain times done, regardless of whether we are aware of it or not. We channel intuition from our higherself, and at times other entities when we are speaking to them. Keep in mind channeling doesn't always need an entity to step through your body, simply connecting to them and communicating with them can be apart of Channeling.
So, why Channel?
It can serve as an important tool, especially if one is interested in working with Otherworldly beings. Channeling can have its dangers but if done properly with measures taken before hand, can prove its uses.
Before getting into Channeling, Its important you have a few basic skills before hand clear methods on grounding, centering, shielding, relaxation and a good basis on chakra work.
Relaxation loosens our muscles and releases stress and tension. Its much easier to channel relaxed then to do it tensed, more info on relaxation can be found on Zebrahs post, Deep relaxation.
Grounding prepares your body beforehand, at times we are so cooped up within our own emotional energies and that of others we tend to overload ourselves with them. Think of it as spiritual hygiene. Not to mention, while channeling we can get lost within our own conscious thoughts mixing them up with the messages we intend on receiving.
~Vibrate your Earth star(below the root) and root chakras, visualize a firm hollow trunk and affirm it to get rid of excess energies and emotions which you not need. Another carrier to this exercise is also to ground the place you are in. Simply visualize the trunk carrying away excess energies down into the earth's core.
Centering brings us to a focal point and allows ourselves to think clearly. Think of your mind as being split up in a myriad different directions, centering is all about bringing it back to a specific place and being in the current moment.
~Ground yourself and relax. Visualize yourself within an eggshell, Slowly extend your awareness out wards yourself and then within. Another way to do this is to imagine yourself as a split up ball, and then bringing the pieces together and making yourself whole.
Shielding, don't think I need to cover much on this one and its importance :P. You will always find yourself more open to connecting if you are protected and More importantly, it never hurts to shield yourself beforehand, as you may not know what kind of lesser spirit you may attract with the vibrations you give out when attempting channeling work.
Now that a few basic skills are covered, time to do the work itself.
Choose whom it is you wish to channel, Ground, center and shield yourself and gradually relax yourself, silently (and respectfully) call out to the entity you wish to speak to and state your purpose; why you are channeling it, etc. It can take awhile before one is able to clearly see or hear the entity they are speaking to, keep trying though.
Its always a good idea to keep a pen and notepad nearby to record conversations you have and then reviewing them later.
Keep a water bottle in handy as well as light snacks for grounding later on. :)
Another point I'd also like to make is that when it comes to channeling entities, especially demons, please do chakra work beforehand. As sometimes their energies can be pretty overwhelming in itself. Also, you dont always need to channel a Deity, Angel, Demon, keep in mind you can also channel spirits, plant devas/elementals, etc.
Good luck and Happy channeling :)