The number one way to learn hauling is to pantomime (a la Mel Krieger), which allows you to concentrate entirely on your arm and wrist motions. Begin with the forearm of your rod arm parallel to the ground (the "ready" position). Put your line hand directly under your rod hand--this rod hand/line hand arrangement is the Beginning Haul Position. Now make a downward tugging motion with your line hand that's about a foot long and stop--this rod hand/line hand arrangement is the Ending Haul Position. Now, move your line hand back up to the Beginning Haul Position. That completes one full haul.
Make another haul, but as once as you've reached the Ending Haul Position, begin to reverse direction and move your hand back up to Beginning Haul Position. The timing and motion is akin to a relaxed "bounce" of your hand and is what is used with the real rod and line (i.e. there won't be robot-like starts and stops when hauling for real). Now let's take that idea and integrate it with a full, pantomimed cast.
Start at the ready position, but with your line hand six to eight inches under your rod hand. Begin to slowly pantomime a backcast, moving both hands in concert. Just before you begin to tip your rod hand back to "turn over" the rod, begin to haul downward with your line hand. As your rod hand continues to move back, mirror its length of motion and speed with the haul. Think: equal and opposite.
As soon as the downward portion of the haul is finished, reset your line hand at the Beginning Haul Position. By the time you get your hand reset, you'll be just about ready to make the forward cast with its haul. If it helps, try saying to yourself "Haul...hand back up." That will be your cue to get your hand back into the Beginning Haul position again.
On the forward cast, just before you begin to tip your rod hand forward to "turn over" the rod, begin to haul downward with your line hand. As your rod hand finishes its final burst of acceleration, your line hand should do the same. And again, your line hand should stop at the Ending Haul Position at the same time as your rod hand stops. At this point you could shoot line or return your line hand to the Beginning Haul Position in anticipation of another backcast (i.e. False Casting).
Work out the pantomimed coordination of the Double Haul slowly at first. When you graduate to the rod and line, be equally restrained. Allow the line to fall to the ground between back and forward casts so there's enough time to get set and ready. Once you're confident in your skills, slowly bring it all up to full casting speed.
Hauling is not overly difficult to learn if you have patience and really practice. Rush things and you'll be frustrated. Take your time, though, and you'll soon be hauling your way to better fishing.
Basic Double Haul Overview