Making alterations to board shorts, or swimming trunks, is a fairly easy process. Board shorts usually have an elastic waistband, in addition to a drawstring. Most pairs are long in length, going down to the knee or the mid-thigh. To take in material means to make the waistline smaller so that the shorts don't hang too far down on the hips. Working with elastic can be tricky, but for the most part this is a good project for those who are beginning to sew.
Measure your waist and hips. If you are not sure how to size yourself, consult a sizing chart. For the most part, if you are two inches smaller than the shorts, then you will need to take in two inches of material.
Remove the cord from the shorts. (Not all styles have a drawstring.) Rip the current tag and seam out of the waistband of the shorts. Remove the stitching that holds the elastic in place. The fabric is usually folded over the elastic and sewn. Once you remove all the excess threads, you will have an extra flap of fabric that is roughly 1 1/2-2 inches that was formerly sewn into the shorts, depending on the style.
Take in the material the required length. Do this by measuring and folding over the excess material, until the shorts are the size that you want them to be. Pin carefully at the waist and along the folded edge (also called a dart). The easiest place to put your dart is on the center of the back of the shorts. Be sure that you are not pinning shut any pockets.
Determine if this is the correct measurement, and make adjustments as necessary. Remember that the pins might poke you, so be careful.
Iron the dart so that everything lays as flat as possible. Be careful of ironing the pins, as they will be very hot. Iron the flap of fabric as well, making sure there is a sharp crease in the material. This helps hold the shape of the dart so that sewing is easier. Otherwise the material bunches up near the waistline.
Sew the top of the dart on the waistband using a strong double stitch. This is the basic straight stitch, depending on your machine's model. Do this twice; once at the top of the waistband and once at the bottom of the waistband. This will hold your dart in place while you reinsert the elastic. Sew one line of stitches that follow the vertical line of the dart to hold the fold in place.
Use the old piece of elastic as a guide to measure out your new elastic, minus the dart measurement. Pin this into the waist band. There should be a slight stretch to the elastic, but not overextended. Fold the flap of fabric back down to hide the elastic and pin again, making sure that the elastic won't slip out from under the two folds of material.
Sew using a zigzag stitch. Use the same number of lines of stitching as the previous seam that you ripped out during Step 2. Sew slowly, removing pins as you go. Be careful not to sew any pins into the waistband. Also be sure to leave room for the drawstring to be threaded through, if necessary.