Hang or position existing lights in the theater in which your play will be produced. Ensure you have enough lights focused on the stage to light it entirely by running a walking test. Turn on each light at the same time, and walk from stage left to stage right with your hand held at eye level. If your hand is not lighted at a given point, reposition the lights so it is.
Light your stage scene by scene. Scenes need to flow into each other smoothly, which requires transition lighting. Work with your actors -- during rehearsal or a scheduled lights-only practice -- to mark each scene in your script with which part of the stage needs to be aglow.
Pick transition sequences using the words of the script. The playwright might have cues already written in, such as "blackout" or "fade to black," which you should heed. These cues often signal a major scene change. Other transitions will be up to you. If a character is crossing from stage right to stage left, consider using a spotlight to follow her while you fade out the rest of stage right so the audience knows which actor to follow for the next scene.
Ascertain the correct gel colors. Gels are inserts placed over the lamp of a stage light to give a glow other than bright white. Reds, purples and blues often signify anger and sadness, while yellows and oranges trigger happier thoughts and therefore signal happier scenes. Gels help create sunrises and sunsets, as well as bright blue skies or gray, rainy days.
Place the gels into the appropriate lamps after you've run through the script. Doing this after a run-through will let you know where scenes take place so you can fit the gels to the lamps used during certain scenes.