Position your two mallets 6 inches apart on the outside ring of your cymbal, loosely holding them. Avoid striking the middle of the cymbal, as it will not vibrate properly.
Softly strike the cymbal, one hand at a time, alternating between your left and right hand.
Pick up the speed of your strikes, from a slow to medium tempo. You will hear the cymbal sounds build from a quiet droning to a dull roar.
Tighten your grip on the mallets, continuing to strike the cymbal at a medium tempo. By tightening your grip, the cymbal reacts to build upon the vibrations, thus creating the roaring sound of the swell.
Raise your tempo from medium strikes to fast while maintaining your grip on the mallets. The slow build will finally have reached its peak and the cymbal will be at its crescendo.
Loosen your grip gradually, and then slow from fast to medium tempo. The cymbal will begin its "decrescendo" and gradually get quieter. This smooth transition from loud to soft should be drawn out as long as possible so that the listener is not startled by the sudden stop of the swell.