Mark the areas on stage where the majority of the acting will take place. The stage director will often be able to provide you with this information, as each show generally has specific "hot spots" on stage where actors are most likely to perform. If there are several different acting spots in a certain portion of the stage, mark the central point between them.
Stand in the active acting area and ask an assistant to turn on the lights above you and focus directly on your head. Ideally, you should be completely centered within the shadow below you. If the shadow seems off, ask the assistant to adjust the lighting until you reach a balance. Lock the lighting unit down so it does not move from this position.
Cut the beam so that it does not affect scenario, audience or any other area that you do not wish to be accented. The lighting should be focused only on the acting area; keep in mind, however, that cutting too much light may darken the stage to an undesirable degree.
Repeat the process for all acting spots around the stage. During acting rehearsals, tweak the lighting until it best suits the show and what you are trying to accomplish.