Determine the "entrance" of the sunlight effect. Will the sunlight be on the stage from the opening of the scene, or will it appear on the stage gradually during the course of the scene, suggesting "the sun is coming out" within the context of the play? If the sunlight is "on" during the entire scene, you can use a lighting instrument with a fixed level of intensity. If the sunlight enters the scene gradually, "cheat" it into the scene slowly on a manual or computerized dimmer.
Establish the "character" of the sunlight. Real sunlight is "white" in color. But there are types of plays where a garish yellow sunlight effect is appropriate, such as comic plays and musicals. The sun in dramatic plays should be portrayed in more realistic tones of white, cool or warm in tone depending on the season of the play and mood.
Decide where the sunlight should originate from and what it should illuminate. A wash across the back of a cyclorama? Sun rays sweeping the stage floor? Should the light appear to be coming from an open doorway, a window, or a specific position in the sky above an exterior set? The former examples, the "washes," are general in location, while the latter examples are specific considerations, or "practicals," that heighten reality. Sunlight washes can be produced by a broad focused instrument, like a large Fresnel, a scoop or a can. Sunlight "practicals" should be produced with focused instruments such as a smaller Fresnel or ellipsoidal spotlights.
Design the shape of the rays. If you want broad, fuzzy rays that wash the set, use a lighting instrument like a Fresnel or ellipsoidal fitted with a colored gel and shuttered or barn-doored narrowly. But if you want crisp, distinct rays, use an ellipsoidal spotlight fitted with a gobo pattern that projects rays on the set. To heighten reality and mood even more, combine your "sun" with a gobo "foliage" pattern projected on the stage walls or floor to simulate leaves casting shadows in the sun.
Give the sunlight "action." There is nothing more interesting than for the sun to appear to be traveling across the sky during a stage play. You can achieve this wonderful special effect by placing several "sun light" instruments across the stage and program a computerized dimmer system to fade the "sun" in and out in sequence in such a way that suggests movement across the stage. Another effect suggesting movement is to create a way for a lighting instrument casting a foliage pattern to slightly move or "shimmer in the wind" during a scene, just as real shadows cast by leaves will appear in nature.