How does one create the 'dreaming body'?

Well, you create it through controlling dreaming, using dreaming in practical ways.  Basically you're creating the doppelganger.  Have you ever had a "dream" where you were looking down on your own body asleep in your bed, or tree, or wherever you sleep?  Well, some people do that naturally from time to time and that is the awareness of the dreaming body which has separated from the physical body.  If you find yourself in this position, move away, go exploring, but never try to wake yourself up.  The problem, in shamanic terms, is that this is still only awareness and not a fully functional dreaming body with purpose and intent.  You gain the dreaming body with conscious effort to use the awareness of it over and over.  Over time and with enough energy it will begin to solidify in a sense into a replica of your own body, but made up of your own personal dreaming energy.  I can't tell you how it works, only that it does, over time.  Lots of time.  Once that body is created it can be projected back into the "real" world.  You're off sleeping somewhere, but your dreaming body is out having fun.  When you wake up tired as though you have been working at something all night, that is a sign the majority of your energy has been ensconced on the dreaming side of your reality.

You said it's made up of our own 'personal dreaming energy' - is that energy accumulated in any unique way?

This is sort of complicated.  The dreaming body doesn't reason; it is apart from that and hiding in the other half of yourself which is asleep.  But the dreaming body in order to act must be able to assimilate a reality, even this one, so it runs back through the rational part of the other side, even when it is separate.  It is accumulated in unique ways through experience with the dreaming body, such as intended dreaming, controlling dreaming, using that energy.  Like exercise, it can't work efficiently if you never use it.  It's something to work on, but pressuring yourself won't help.  You have to chop wood, carry water.  Shamans are built from the ground up, details, discipline, work, etc.  A shaman without the work would be pretty useless.

The dreaming body can do what the physical body cannot in terms of illuminating those lines within ourselves which connect to aspects of realities we cannot endure with the physical body.  Once the two are joined, assuming you have been doing the same on the physical side, something has to give.  All your awareness is lit up and becomes too large for your current living arrangement.  And you burn.