Whether you wear jeans or slacks, there is no need to fret if you put on a few extra pounds. At least, not when it comes to keeping your trousers fitting comfortably. That's because making the waistline bigger is a moderately simple process.
Measure your waist. This will allow you to know how much you need to expand the waistline in your trousers. For instance, if your trousers size is 70 cm (28 inches) and your waist measures at 75 cm (30 inches), then you will need to increase the waistband by 5 cm (2 inches).
Find the spot on the waistband where you will add an extension. Adding extensions to the sides of the waistband is generally considered the most effective method.
Divide the difference. If your waist is 5 cm (2 inches) bigger than your trousers size, then you will need to plan to make your waistband bigger by 2.5 cm (1 inch) on each side.
Using scissors, cut the elastic tape 1.2 cm (1/2 inch) longer than the extension needs to be. Sticking with the idea that your waist is 5 cm (2 inches) larger than the waistband, cut the tape 6.2 cm (2 1/2 inches). If your waist is 2.5 cm (1 inch) bigger, cut the tape 3.7 cm (1 1/2 inches), and so on.
Mark the portion of the waistband where you plan to place the extensions. If you are increasing your waistband by 5 cm (2 inches), or 2.5 cm (1 inch) on each side, mark off an area half of that on the waistband -- 1.2 cm (1/2 inch) on each side.
Cut the marked section on each side of the waistband, forming a "V" with the cut -- start at the top of the waistband and stop at the seam. Do this on both the inside and outside layers of your trousers, and on each side.
Place the elastic tape on one section of the waistband, 6 mm (1/4 inch) from the top of your V cut. Pin it in place, then use the other end of the tape to pin it to the other side of the V cut. Do this one side for now.
Sew the elastic tape to the waistband, starting at the top of the tape and working your way to the bottom. Follow this process on both sides of the V cut.
Repeat the above two steps on the other side of your waistband. Once you've done that, try on the trousers. If you've followed the above measurements and made the right cuts, they should fit comfortably again.