How do you lose 10 pounds in three days?

Trying to lose 10 pounds in three weeks is a bit too simplistic. Its also a bit on the high side. Most doctors and nutritionists agree that 1 or 2 pounds a week is more reasonable. Its also a bit shortsighted - you need to concentrate on long term goals, not that bikini you want to get into, or something like that.

Most importantly, you must diet and exercise. You can not succeed by only doing one; you must do both.

As far as specific details, that is highly dependent on you and your current situation, so it does not make sense to give any details. Talk to your doctor, and make sure that you are physically able to exercise, and set some goals. Keep in mind that in order to lose more than just "water", you need to tap into stored fat, and that requires 40 minutes or so per day of exercise with your heart rate in the target zone, again, as specified by your doctor.