Sweatpants that fit right are generally described as just about the most comfortable clothing item you can find. The reason sweats are so comfortable is the stretchy fabric forgives a slight weight gain without digging in or chafing your skin. If your sweat pants are too big, though, it can be a problem. The knitted fabric tends to be so stretchy and forgiving that oversized sweatpants will just fall off. A little bit of tailoring work will make your oversized sweatpants fit again.
Measure the waist and hips of the person who will be wearing the sweat pants by wrapping the tape measure around them. Divide each number by 4. You need to divide by 4 because you will be taking a portion of excess fabric from each of the four pieces of fabric that make the sweatpants.
Turn the sweat pants inside out.
Mark the waist and hip measurements, with the tailor's chalk, using the measurements you just calculated in Step 1. Make the marks just inside of the outer seam lines of the sweatpants. It will be easier to sew if you sew the hip seams and avoid sewing the deeply curved areas of the trousers.
Use the side of the French curve that is marked for hips to draw the rest of the new sewing line. Place the French curve so it lines up with the marks you made and then fill in between them.
Have the person who will be wearing the trousers put the inside-out trousers on and have them adjust the crotch depth so it fits comfortably. It is important to check the crotch depth when making trousers smaller because if it does not fit right it makes the trousers very uncomfortable.
If the crotch depth is right without altering, go on to Step 8; otherwise mark the spots where the top of the waistline needs to be with the tailor's chalk while the trousers are still on the person who will be wearing them. Get the trousers back after you have marked the waistline.
Rip out the stitches in the old waistband by slipping the pointed edge of the seam ripper under every other stitch. Keep the elastic.
Put the ballpoint needle in the sewing machine according to the machine manufacturer's directions. Ballpoint needles prevent knitted fabrics from fraying on the seam lines because they do not cut the fibres of the fabric.
Sew along the tailor's chalk lines you drew down the outer edge of the sweatpants. If the overhang is more than a 1/2 inch, cut the excess off with the scissors.
If you ripped out the waistband, measure the width of the elastic and add a 1/2 inch to that measurement. Measure that far outside of the line you marked in Step 6 and make a new tailor's chalk line.
Fold the edge under 1/4 inch then fold the waistband again on the first waist line you drew. Pin the folded fabric down with the straight pins.
Sew all of the way around the sweatpants except for a 2 inch gap. You will need the gap to feed the elastic back into the waistband.
Look at the waist measurement you took in Step 1 and cut the elastic to that size.
Attach a bodkin or safety pin to the end of the elastic and feed the elastic through the opening. Make sure the unpinned end does not go into the opening or you will have to start over.
Sew the ends of the elastic together by stitching back and forth over them at least 3 times. Sew the gap in the waistband casing closed.