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To create a casing for your elastic, you will need to hem your pants so that the elastic will have somewhere to go through. Start by folding over the bottom of your pants twice. Make sure that the area you fold over is wide enough for your elastic to fit into. Hold your elastic up against it to be sure. Next, sew along the edge of the hem to secure the casing. Make sure to leave a small gap in the opening that you can insert the elastic into when you are done. If you are inserting elastic into a pair of pants that has already been completed and has a wide enough hem to fit elastic into, then you will need to cut a small hole into the inside of the hem. Use a pair of scissors to snip a small slit across the inner layer of the hem and use this opening to insert your elastic into. When your casing is complete, attach a safety pin to the end of the elastic strip and secure it. Attaching a safety pin to the end of the elastic strip will provide an anchor for your elastic and make it easier to work it through your pant legs. Make sure that you attach the safety pin far enough away from the end of the elastic so that it will not slip out. You may even want to insert the pin through a couple of places in the elastic to be certain that it is secure. To work the elastic into the hem, insert the closed safety pin into the opening in your pant leg hem. Then, begin to work the elastic through the hem using the safety pin to move it along. You will have to work slowly and move the safety pin through the hem just a little at a time. This may be easier or harder depending on the size of your casing and the material of your pants. When you get to the end, pull the safety pin out the other side and then remove it. Sew the two ends of your elastic band together by hand or using a sewing machine. Then, use the sewing machine or a needle and thread to close up the opening that you inserted the elastic through. Repeat the same process for the other pant leg.
Fold over the bottom of the pants twice. Sew along the edge of the hem. Attach a safety pin to the elastic. Work the elastic into the hem. Sew the elastic ends together and close the gap.