The belt vessels form a connection between Heaven and Earth, through man.  The ascending channel originates with Earth energy, somewhat below the feet, and spirals up through the body (in much the same proportions as a belt) to somewhat above the head.  The descending channel originates with Heavenly energy, from above the head, and spirals (in a direction opposite to that of the ascending channel) down through the body, to below the feet.

Figure that, in a standing posture, you're already aligning with the forces of Heaven and Earth: you simply employ those forces to do the work of activating the vessels.

In formal description the belt vessels have a steady formation.  They repeatedly intersect the governor (the back of the microcosmic orbit), functional (the front of the m.o.), and thrusting vessels (one vertically through the center-left of the body, one through the center-right, and one directly in the center).  Since each thrusting channel goes down through its respective leg, the single circle of the belt channel encompasses both legs.  (It doesn't split into a separate circle for each leg.)  The center-most thrusting channel (the core channel) doesn't get intersected by the circle of the belt vessels, but does - to a certain extent - get activated by them.

In beginning practice, however, the circumference of the spiraling varies a lot. You'll find that the spirals shift to various layers of the body, from outside of the skin to right into tendons, joints, bones.  Sometimes the spirals are very tight and focused, other times they are broader.  This variety happens both as a result of your intent and, often, as a quite spontaneous expression of the energy.  This is all fine; the belt vessels are particularly safe to work with.

Later, as you develop the thrusting vessels, du & ren (the microcosmic orbit), the core channel, and the energetic centers (along the core), the belt vessels naturally resonate and relate with them.  As that occurs, the belt vessels lend stability to that whole energetic structure, a certain amount of protection, and assist in various nourishing blendings.  The microcosmic orbit tends to get more than its share of emphasis, in my opinion: The pieces to this (thrustings, core, centers, du, ren, belts) are all important, and come together to make something very helpful - well worth attention and development, over time.

Spiraling really gets through blockages in your system, much as a drill goes through... anything.  As well, it blends energies.  Having the belt vessels go through your body bottom to top, and top to bottom, really blends and evens out your whole system - which is very healthy, and provides a relatively gentle, balanced, integrated way to approach the energies of Heaven and Earth.

The Heaven - man - Earth hookup is important and beneficial, in a number of ways.  Humans have a basic, deep need for connection to the energies of Heaven, and Earth.  If left unfulfilled and undirected, that hunger expresses in all sorts of twists and warps.

Additionally, the fact that Heaven and Earth are benevolent forces, that are much bigger than the elements within the human body... It's very stabilizing, and nourishing, to the forces within the body, to connect with Heaven and Earth at either end.

It's easy.  I can't think of any other single practice that, once established, sort of "does itself" and addresses your whole body in one comprehensive balanced sweep.

You needn't do the belt vessels all at once; they're something that you can sneak up on.  You can do spiraling work on parts of your body, with variations, and do the full sweep as you feel ready.  

There are plenty of spiraling techniques in the Iron Shirt Chi Kung 1 (pg. 98-99, 105-106, ...) and Bone Marrow Nei Kung (bone breathing, bone compression) books.  (Also in the videos: V57 and V58.)  Be careful to just spiral energetically; you can injure your knees and ankles if you try to physically twist them.

Assume any standing posture (for instance, Embracing the Tree).

Spiral energy clockwise down (one to several feet) into the ground.  After a brief pause, the Earth naturally responds by spiraling energy back up, counter-clockwise.  Let that energy spiral up through your legs, torso, head, to a little ways above your head.

Once the spiral is above your head, pause, and accept the energy of Heaven as it spirals down, clockwise.  Let that energy spiral down through your body down into the ground.

On the way (both ascending and descending), as the spiral passes each of the three main centers (power, love, wisdom), spiral in to the light of each deep-center.

Repeat the process as many times as you like.

CLOSE

To close the meditation, it's traditional to gently gather some of the focus and energy in your lower tan tien.  Be attentive to do this in a way that's harmonious with the heaven-man-earth spirals that you've generated.  There's no need to entirely close those down; they're beneficial and protective.