A weird question, isn't it? But when I search it on the internet, the result did surprise me, for that there are such many people wondering about this. So I put the best result from the several sites here, wish this could help you…
Choose an appropriate time, but not at the centre of an over-populated hall, you need to create a kind scene first, to get people's attention. The aim of this is to make the people believe that you really fainted, and not to be stepped on and killed.
Initiate steadily downwards while walking, and gently hold your head as if you have a severe headache, close your eyes a bit, but make sure you can still see partially; but don't forget you must not overdo this.
Stay quiet. You don't have to do this suddenly, but slow down gradually until you stop. Close your eyes and open for a while attempting to focus.
To make it realistic, you must appear dizzy and weak for a while before going down. Drop your focus slowly on what you are doing, and appear increasingly weaker. Take your breaths slowly, while you become weaker.
It gives you an odd feeling and you should pretend to need some fresh air or a glass of water. You can either say you wish to sit down or ask someone nearby to open the window. Sit awhile and stand up slowly, and wait for a while before falling onwards. But don't fail to start a sentence such as, I just.....you don't have to finish the sentence.
You must be cautious while falling in order not to hit your head on a hard surface or hurt any part of your body. If you are standing, get your knees folded and make sure they hit the floor before trying to make your upper body fall. You must be fast while carrying out each of these tasks to make the whole thing appear real. Try falling on your thigh's backside, do not use your tailbone or hip ,and then drop the upper part of your body quickly.
Stay down on the floor and keep reminding yourself of the fact that you are not flexible in any way. If anyone tries to raise up your arm and shake it, allow your body to relax totally, and when the person drops your hand, permit it to fall without trying to control the fall.