Figure out what kind of illness to fake. Usually choose the kind of illness that is not easy to work through and isn't serious enough to go to the doctor for. Usually a cold, fever, or a minor illness that lasts 24 hours are good choices. Be aware of the signs of illness and don't exaggerate your performance. Be anticlimactic in demeanor. The patient will not be smiling and in high spirits. You need to act distracted.
Mention offhandedly a day in advance that you are showing symptoms of illness. If you don't want to go to school on Monday, act depressed and disoriented on Sunday. Tell your surroundings that you don't feel well or are a little lightheaded. Lose your appetite and go to bed early. This will make it more believable the next day when you actually have an "attack".
Think about past illnesses. People know what your past illnesses were like, so think about how you felt then and how you behaved then. Then recreate the symptoms and emotions of the past. It's easier to pretend an old illness is coming back than to pretend it's unfamiliar to you. How to look pale. Putting green concealer on your cheeks and forehead can make your face look pale. Don't apply too much green makeup, as the saying goes.
The demeanor should be anticlimactic. The patient will not be smiling and giddy. You need to act distracted. If you're one of those people whose demeanor is cranky when you're sick, act like it. Don't appear to be enjoying yourself even when participating in your favorite activities. When invited to watch a favorite movie, for example, politely decline people's requests.