I'll go ahead and try to answer this with one of my lessons.
The Process of Astral Projecting
Stay relaxed, and keep your mind focused.
Once you feel that little tug behind your eyes, remain relaxed, focused, and still. That's the trick to the simple way of projecting, and it takes time. For some it's very difficult to do, so I suggest meditating first or running around or anything that will help you calm down and relax.
You do not have to see your astral body actually separate. The process itself is simple. When you astral project, your astral body, along with the lesser subtle bodies go with it. This separates the various forms of projection. Try working your way up the ladder. Try mental projection sometime if you haven't already, get used to that, then try astral projection. It will be so much easier.
First your bodies bodies begin to level off on different planes, causing for some, a vibrating sensation. There are 2 planes directly on top and below this plane, the physical plane. They are not spaced out like most other planes, in fact something can exist on either at the same time. So your astral body, when first projecting but not completely lifting off just yet, remains in the planes either directly above or below this one. This gives the notion of not separating yet, but really your bodies are already almost there. All you must do from here is to create a connection, which typically comes naturally but some people have to do it consciously, and climb. Climbing is the actual case of projecting further into the astral planes, this may take some practice when projecting consciously.
Everyone astral projects, but not everyone is aware of it. It really is a naturally occurring habit that one can with practice learn to do voluntarily.
To help with climbing I suggest trying dream control, it works wonders for you when you start projecting.
Hope this helps.