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Try on the pants and pinch the excess fabric at the waist until the waist is as tight as you like. Use a tape measure to see how much fabric is between the tip of your fingers and the inseam. For example, if you need a small adjustment, you might only need to remove 1 inch (2.5 cm) of fabric. For a larger alteration, you could remove 2 to 3 inches (5.1 to 7.6 cm). Take off the pants and use a seam ripper to pull out all of the stitches that are securing the back belt loop to the waist of the pants. Then, remove the loop and set it aside. Keep the back loop so you can reattach it to the pants after you've adjusted the waist. Use the seam ripper to remove the stitches from the middle of the waist on the back of the pants. Remember to start in the center and remove an equal length of stitches along both sides of the waist.  For example, if you wanted to take in the waist by 2 inches (5.1 cm), you'll need to take out the center 2 inches (5.1 cm) of stitches on the back of the waist. Once you've removed the stitches, you should be able to open up and unfold this part of the waistband. Fold the inside-out pants in half so the creased pant legs are stacked. Then, pinch the excess fabric along the waist that you want to remove. For example, if you want to make the waist 2 inches (5.1 cm) narrower, pin 1 inch (2.5 cm) of the fabric from the center seam. Since the pants are folded, this will mean you're reducing the waist by 2 inches (5.1 cm). Start stitching where you pinched the fabric together. You will probably need to remove pins as you sew so you don't stitch over them. Straight stitch until you reach the bottom of the waist.  You can use a sewing machine or stitch the waist by hand.  Once you've sewn the waist, try on the pants to see if the waist fits better. Keep in mind that the excess fabric will form a small loop inside the waist of the pants. If you're happy with how the waist now fits, fold the waistband back down. The waistband stitches should line up with where they were before you seam ripped them out. Then, sew the waistband back in place and reattach the back belt loop if you like.  Turn the pants right side out before you're ready to wear them.
Decide how much fabric to remove from the waist. Remove the belt loop from the back of the pants. Seam rip along the back of the waist near the middle. Turn the pants inside out and pin the excess fabric from the center seam. Straight stitch from the top to the bottom of the waist. Fold the waistband and straight stitch it closed.