Choose a suitable time, this would not be in the center of a crowded hall, until you create a sort of a scene first, and acquire people's attention. Your aim is to get individuals believe you have fainted, not get stepped on and actually get killed.
If you are walking initiate steadily downwards, and hold your head gently as you have a headache, squinch your eyes and see a little; but remember don't do this too much.
Keep yourself quiet. Although not abruptly but just slow down steadily until you stop. Open and close the eye and squinch for some time as you are trying to focus.
To keep it realistic, you have to look weak and dizzy for a moment prior to go down. Slowly drop your focus on whatever you are working on, and turn out to be weaker and weaker. Take slow breaths, as you're going too weak.
It seems like you feel sort of odd and you should pretend that you want a glass of water or some fresh air. You can either ask someone to open a window for you or just say that you need to sit down. Sit for a moment and get up slowly, then hesitate a little and fall onwards. But keep this in your mind that prior you do all this say something like "I just...", be careful that you don’t complete your sentence.
You have to keep this confirm that you drop yourself safe and sound, without hitting your head or harm your body. If you are in standing posture, fold your knees and make them hit the ground before you try to fall your upper body. Do all this action very fast otherwise this will seem fake. Attempt to fall on the back side of your thigh, not your hip or tail bone and then quickly drop your upper body.
Lie down on the ground and keep this in your mind that you aren't inflexible. If someone try to lift your arm and shake it, let your body totally relax and when they drop it, just let it fall as it would.