Shake your body, shaking out your legs and arms so that you're relaxed and loose.
Walk casually across the stage, smiling and whistling nonchalantly as you go. Take three normal-sized steps forward.
Step with your right foot forward on the third step and hook your left foot behind the heel of your right foot. Fall forward as you do so and land hard on your left foot, catching your balance before you fall to the floor.
Make an expression of surprise as you trip and let your body lingo show that you were not expecting this to happen. If on stage, look at the audience in disbelief and play off their reactions.
Practice tripping in front of a mirror or in front of a friend so you can get it to look as realistic as possible.