You want to do it slowly and gradually. Your belly is not supposed to be flat just after you've had a baby. A lot of the exercises that you would normally do to tone your stomach, like crunches, sit-ups, and planks, are not suitable immediately after pregnancy since they put too much pressure on the abdominal wall. You'll want to wait until your body feels ready before you start doing these types of exercises, which could take 6-10 weeks.