My good friend and Tai Chi Master, Alan Lumder, wrote about rooting:
Rooting is fundamental to effective technique. The Chinese say heaven and earth which means that the legs are solid and the upstairs moves lightly. Rooting can be both static (fixed) and moving. It is best for your students’ to start developing their roots with standing exercises. You may want to consider the following:
Have your people start with fix postures. Use holding the ball or a move from your forms. Start with holding the move for 1-3 minutes. Let them feel the energy drain from upstairs and settle into both legs. It is a process of relaxation. Make sure that their posture is good. Knees must not exceed the toes. After a while they can visualize energy orbits. They should work up to 15-30 minutes. By holding postures, they can work out the energy movements. In time the entire body becomes light and balanced.
When they can hold fixed postures, work on their forms. By doing the forms slowly, using empty steps, and proper body mechanics, they can develop moving roots. The need to be focusing on their legs, where they are now, and much less on destination.