In this demonstration, I used a pair of jeans from the men's section at Walmart!
Step 1: Try on the jeans and pinch where you want to make them tighter in the waist
Step 2: Mark with a sharpie where you want to take it in
Step 3: Use straight pins to mark the section off
Step 4:mark a line along where to sew and sew on the line.
Step 5: repeat steps 2-4 on the other side of the new seam.