The dreaming body is accessible through any of the levels of dreaming; however, the most useful level of access is found after coming through the second level, to the third, when dreaming is no longer the fantasy factory it is on the first two.
You can encourage the creation of a dreaming body, in fact, by working at the process of splitting awareness - being in two places at once, or being in multiple places at once, across time and distance.
Though, the latter tends to result in vertigo the first times you practice it. I've talked about splitting of awareness before, but let's be more practical and try to relate this back to the dreaming body.
Journeywork and Splitting Awareness
First, we define terms. What many people refer to as journeywork I refer to as splitting awareness. The way I work with it is considerably different, yet it springs from the same energetic foundation. My tradition was more practical after a time about that movement of awareness - long complicated journeys left you too exposed - so they learned to shift rapidly in just about any direction with considerable control. It is still a journey, it still requires trance, but it works on levels which are truly split so the 'mask' of normalcy you wear is hardly ruffled at all. Those that have dealt with me in person I think probably understand that better because they have seen it work, if they remember it, anyway.
What will the person next to you at that stoplight you always stop at be wearing? Or what kind of car will they be driving? This takes you out of normal awareness altogether.
The forceful splitting of ones awareness causes us to be able to move in ways which are very similar to the way in which we need to learn to work with the other side of our energy in dreaming, and therefore, create the dreaming body. If you're a crappy dreamer, you can still work at creating the dreaming body from a conscious position of learning to shift and split your awareness. Many psychics, readers, etc. do this on a very elementary level. Shamanically, however, the movement is complete, radical, and, I realize for many, upsetting in the beginning because we fear so many things, especially the thought of giving up any control. The people I have trained as healers already have done this to a great degree. You can't step in to someone far away and affect their healing process without a splitting of awareness. That is only, of course, on a healing level; there are other levels which involve many different sorts of work and or seeing.
The forceful splitting of ones awareness causes us to be able to move in ways which are very similar to the way in which we need to learn to work with the other side of our energy in dreaming, and therefore, create the dreaming body.
My point, even though I've taken far too many words to get to it, is that we can force the development of the dreaming body from outside dreaming; yet, in a weird sort of confluence of personal energy, it must emerge from within dreaming - the side of our energy that is not yet recombined with waking reality.
Methods of Splitting Awareness
Now I want to delineate a few methods. One: practice movement of your awareness outside yourself and learn to play with it. Look at a paperclip on your desk and move to a different perspective of being behind, under, inside the paperclip. Look up and back at yourself. Change from the paperclip to moving to different places in your apartment, or house, etc, but still within sight of yourself. This is a shift in perspective; its simple, non-threatening, but it starts a process that can then move forward to more complicated shifts and ultimately to completely splitting the awareness.
Two: graduate to places in your room/apartment/house, still within sight of yourself. Look back on yourself. Doing this awake, rather than asleep, is not a problem because it is just splitting the conscious awareness.
Third: expand this practice outside your immediate surroundings. This requires you to find things to look at that can be verified by going and looking.
Four: once you have enough verification experience and shifting experience, challenge time. Shift your awareness into the future, the past. What will the person next to you at that stoplight you always stop at be wearing? Or what kind of car will they be driving? This takes you out of normal awareness altogether. From there, you are free to move your awareness in any of the directions. This, you might notice, also corresponds to the four levels of dreaming, but with slightly different perspectives.