Use a measuring tape to measure around the part of your torso where you want the belt to sit. Be sure to measure over the clothing you plan to wear with the belt.
Keep the tape snug but not tight — it shouldn’t dig in, and it shouldn’t hang loose.
If you don’t have a measuring tape, you can use a piece of string instead. Wrap the string around your waist, mark where it meets, and then measure that length with a ruler.