Hemming the waist of a piece of clothing means taking in the garment so that the top, dress, or pants will be smaller at the waist. The easiest way to take in the waist of a garment is to reduce the width at the side seams. Hemming the waist can help you get more life out of a piece of clothing if you have lost weight, if the garment has stretched out, or if you would like to re-size a piece of clothing for a smaller person.
Put on the garment inside-out, with the seams showing.
Pinch the top or bottom edge of the garment that is the widest on your body, taking into your fingers the amount of fullness that you'd like to hem, the side seam centered on the fold, and place a pin at the top edge through both layers of fabric.
Pinch the side seam at the place on your waist that fits the tightest and place a pin at the point on the seam where the garment fits you, usually the bust or the hip.
Take off the garment and place it on a table.
Use a pencil and ruler to draw a straight diagonal line from the pin at the area of most fullness, which you marked in Step 2, to the area of least fullness, which you marked in Step 3. The line you draw should dead-end into the side seam at the point of least fullness.
Sew along the line you drew using a machine straight stitch.The stitches should intersect with the side seam stitches.
Repeat the above steps with the other side seam of the garment.