Tightening a dress around the waist is pretty easy. You will need simple pins and a mirror (or someone to help you out).
Steps

Step 1. Put the dress inside out

Step 2. Place your hands on each side of the waist and grasp the fabric you wish to tighten
Grab the same amount on both sides.

Step 3. Put the first pin where you need to pick up most of the fabric

Step 4. Place the pins above and below the side seam until you get a good fit

Step 5. Take off the dress and baste it

Step 6. Put it on again, this time in the right way, to make sure you get the cut you want
Make changes if you are not satisfied with the result.

Step 7. Machine sew the dress along the line of the basting

Step 8. Remove the basting stitches

Step 9. Try on the dress again

Step 10. Cut off the excess fabric, leaving about 2 cm

Step 11. Press the seam open to get a good finish

Step 12. Done
Warnings
- When you finish basing, make sure the new seam disappears at both ends of the change.
- Be careful not to prick yourself with pins when you take off your dress. If this happens, be sure to apply an antiseptic to the affected area and cover it with a cotton ball, trying not to stain the dress.