Inspect the lighting fixture. Theater equipment is often subjected to quite a bit of wear and tear, and it is much easier to deal with a broken fixture on the ground than one that has already been hung. Make sure it is structurally sound and no parts are missing or broken--if any are, you will need to repair or replace them before hanging the fixture.
Rotate the fixture's C-clamp into its proper position. The position is determined by the direction in which the light will be pointed--if it is roughly parallel to the pipe on which you are hanging the fixture, the C-clamp will need to be parallel to the bar on which it is mounted, whereas if the light will be perpendicular to the pipe, you should orient the C-clamp perpendicularly to the bar. To rotate the C-clamp, you must first loosen the small bolt on the side of the clamp, then re-tighten it once the clamp is in position. Finger-tightening this bolt is usually preferable to using a wrench, as the small bolts break easily and the bolt itself does not support any weight.
Place the clamp on a lighting pipe with its tightening bolt toward you. You may have to open the tightening bolt very far in order to place the clamp, depending on the diameter of the pipe. Make sure the clamp is all the way on the pipe.
Tighten the tightening bolt. The end of the bolt will press against the pipe to secure the fixture. Use a wrench to tighten this bolt, as only finger-tightening it is unsafe.
Attach the fixture's safety cable to the pipe. Loop the cable through the bar the C-clamp is attached to, then around the pipe, and clip the cable's ends together. This cable provides a safeguard against the fixture falling if the clamp should somehow fail.