Taking In Pants By Adding Elastic
Most pants have a foldable waistband, you can slide a length of elastic and securely, turning a normal waistband into an elastic waistband. This is easy to do. I use a sewing machine, but you can sew it by hand quickly.
Open a vertical slit on one side of the waistband, just enough to pass through a layer of fabric, and put your rubber band (3/8" to 1" wide rubber band works best). I cut a slit on the outside of the pants, but it will be less noticeable on the inside. Cut a matching slit on the other side. Note-the waistband of some trousers is stitched vertically, which will be a dead-end of your elastic band. If they are just for decoration (such as brand labels), you may need to cut them off, or add some elastic bands at the back, or add some elastic bands in several areas to achieve the best effect.
Tuck your elastic for pants into one slit, slide it all the way and pull the other out. I fixed the safety pin to one end so that the rubber band can pass through. Make sure you did not pull the rubber band from the starting point. If the ends of the rubber bands are close to the edges of the length, pin them to prevent slipping back.
Use a coordinating thread to sew your rubber band next to the seam where the rubber band is located. I ran a straight machine stitching back and forth vertically several times to keep the elastic safe. Trim your elastic band so you can tuck a short band into the slit and then cover it around the slit. This can hide the slits and prevent wear of the pants, so be sure to leave a little space on both sides of the slits.
Ask the person wearing the pants to try them on, pull the unfixed end of the elastic to the desired tightness, and then fix it with a nail. Sew the elastic and rip on the other side (be sure to remove the pants from the wearer before putting them back on the sewing machine!) and that's it!
In order to be more concealed, or without a machine, you can make a slit in the pants. Slide the elastic band over. Secure the elastic. Depending on how much you need to take in and the pants fabric, you may get away with only sewing on the fabric inside the pants, but more likely, you need to sew both layers to ensure the safety of the elastic. Then you can use a whip to sew through the waistband fabric inside, or leave it open and use a product.