The easiest ways to do this are:

	Lie flat on your back on the floor. Place one hand on your belly, and one on your chest. Notice which hand moves more as you inhale and exhale.
	To belly breathe, focus on keeping the upper hand still and moving the hand on your abdomen up slowly with each inhale, and down even more slowly with each exhale. The following breath counts can be useful: the 4-4-2 (inhale for 4 seconds, exhale for four seconds, then pause for 2 seconds before the next inhale), the 5-5-3, the 6-6-4, the 7-7-5, the 8-8-6, and so on.
	You can also try the three-part breath, which begins with abdominal breathing and ends with the chest. Inhale into your lower belly, then into your mid-belly and abdomen, and, finally, into your chest and upper lungs. Exhale from the chest, abdomen, then lower belly in sequence. You can place your hands on the various segments of your torso as guidelines.
	It also helps to picture your belly as a balloon you're trying to blow up with each inhale. You can slightly exaggerate distension of the belly when inhaling and pull the belly in a bit when exhaling, to get the hang of the motion.
	When my torso tension is at its worst, and I'm struggling to get a good breath, it helps me to lie face-down on the floor and press my belly out as far as possible. This takes effort and is not a resting position, but it can help slow your breathing. Alternately, you can place a bolster under your hips, belly, or chest, as well (firm pillows or rolled-up yoga blankets work nicely).

The best resource I've found for problems with hyperventilation is Chronic Hyperventilation Syndrome: Recognizing and Correcting your Breathing-Pattern Disorder, by Dinah Bradley. You can order it off the web."