Set your lighting instrument (fixture) on the table before you. A theatrical lighting instrument consists of three main components: 1) the body, 2) the barrel and 3) the yoke. The body, the back part of the fixture with the plug-in cable sticking out, houses the lamp (bulb) and reflector.
The barrel slips into the body toward the front of the fixture. It holds the lens assembly. The barrel is held in the body with a large wing nut-like handle that can be tightened or loosened to allow the barrel to slide forward or back within the body. The yoke is the "U" shaped handle attached to the sides of the fixture's body. The yoke is attached to the body on each side with a large wing nut-like handle, which can be tightened or loosened by turning.
Hold the C-clamp by the C shape in one hand and loosen the bolt at the bottom of the clamp with a crescent wrench. Remove the bolt completely from the clamp.
Insert the bolt up through the hole in the yoke from the inside (next to the body). Spin the C-clamp down on the bolt until it is snugly set to the yoke. Tighten with the wrench.
Using a ladder or lift (if needed), take the fixture to your hanging batten (light bar) or truss. Place the C of the C-clamp over the bar. You may need to open the clamp a bit by turning the smaller bolt on the C with the wrench. Once the C is over the bar, tighten it down until it closes. Let go of the instrument until it hangs freely. Tighten up the small bolt so it is firmly gripping the bar.
Open the safety cable's carabiner clip. Wind the safety cable through the yoke and over the the bar. Attache the clip back in place. Loosen the wing nuts on each side of the yoke until the yoke moves easily but with some bite. Pull the four shutter fans out from the barrel. These are four identical handles toward the mid section of the barrel--one on each side.
Plug the light into a power source. When the instrument is lit, point the nose (front) toward the place you wish to light on the stage. Hold it in place with one hand while you tighten the yoke wing nuts. Wear gloves to avoid burns.
Twist the wing nut on top of the body until it is slightly loose. Now grasp the barrel and slide it in and out to adjust the focus. You will notice that the beam's projection softens and widens or narrows and hardens as you move the barrel.
Tighten the wing nut back up when you have the desired focus. You can now further shape the light by moving the shutter fans in, out, up and down as desired.