Ideally you want something that incapacitates you from major responsibilities without being so serious that others will take you to the doctor or hospital. A cold, fever, or 24-hour bug are all good options. Make sure you know the symptoms you want to fake, and limit your performance to just those. If you do tons of irrelevant symptoms, this will not work the way you want it to. If you're wanting to stay home from school Monday, act tired and sluggish on Sunday. Say you aren't feeling well, or have a slight headache. Don't eat much and go to bed early. This way when you start showing more severe symptoms it will be more believable. You've been sick before, and people have noticed. Think about what it felt like, and what other people noticed the most when you were sick? Try to replicate those symptoms and channel that feeling. It'll be much easier to convince people that you have come down with something you have had before, rather than tackling a whole new ailment. If you have green concealer, rub it on your cheeks and forehead to make you appear pale. Don't paint your face green, just slightly change the color of your skin. If helpful, use a little bit of powder or flour.   Make sure you know how to apply makeup effectively. If you are obviously wearing makeup, you are sure to be caught. If you use makeup, try to avoid being touched. You'll be found out if someone puts their hand on your face and the concealer comes off. Walk slower, with shorter steps. Take your time when getting out of bed or a chair. When you stand up at your desk, pretend to lose your balance a little bit and put your hand on your desk to "regain" your balance. To remember what being dizzy feels like, wait until you are in private and spin around until you get a little dizzy. Make note of how it feels and how you're acting. When you're in front of others, replicate that behavior, but only slightly. People who are sick don't feel good, so don't joke around and laugh and smile too much. Give people the impression that you're disoriented...

TL;DR in Vietnamese:
Quyết định căn bệnh mà bạn muốn giả vờ. Bắt đầu nhắc đến triệu chứng bệnh vào ngày trước khi bạn muốn giả vờ bị ốm. Khơi gợi lại ký ức. Tạo vẻ nhợt nhạt cho khuôn mặt. Hãy giả vờ như bạn đang cảm thấy choáng váng và xây xẩm mặt mày. Tỏ thái độ không thoải mái. Tỏ vẻ đờ đẫn. Hãy hành động như thể bạn cảm thấy khó chịu khi bị ốm. Không nên bất ngờ trở nên khoẻ hơn.