Create the impression that you are whispering through your body language. Hunch over and tip toe across stage as if trying to be quiet. Exaggerate every movement to ensure the audience understands what you are trying to convey.
Show the audience that you are whispering in the scene by your facial expressions. Raise your eyebrows and put a finger to your lips to shush others in the scene. Always turn toward the audience when doing this instead of turning away no matter where your fellow actors are standing in relation to you.
Project your words in a raspy voice as you speak while pronouncing every word clearly. Practice using different sounds with your voice until you've reached a voice that best sounds like a whisper.
Practice whispering to make sure you are articulate as you do a stage whisper. Whisper vowel sounds and then whisper these sounds with a consonant to practice pronouncing words clearly. Work on making it sound as if all sounds are on the same pitch as you whisper.
Do your stage whisper on stage before your production or in front of a friend to see if you are doing it correctly. Make sure you can be understood and heard while you are whispering.