For years, I refused to do this alteration because the outcome was never up to my standards. After considerable begging on the part of my small-waisted clients, I came up with a method that works much of the time.
Jeans present some very specific challenges, first and foremost, the flat-felled seams--those iconic heavy, top-stitched seams that many a sewing machine refuses to sew through. The center back seam is always done that way, and attempting to re-sew it to take the pants in down the center back never looks right. Secondly, many jeans have a metal rivet at the side seam where the edge of the pocket meets the side. These rivets cannot be removed without damaging the fabric, and thus you cannot take the jeans in at the side seam. The third challenge is the upper back yolk of the pants, which can be rather narrow depending on the height of the rise. So what's a seamstress to do?
My alteration will only work to reduce the size of the waistline itself. It will not reduce the hip area.
STEP 1: Using a heavy T-pin, I make a pin tuck through all the layers of the waistband of the jeans while they're on the client. This lets me know how much I will need to reduce the waist.
STEP 2: Make a heavy chalk mark on the inside of the waistband right across your pinned tuck. Remove the pin, and carefully measure the distance between the two chalk marks. Let's pretend it's 3".
STEP 3: With a different colored chalk, mark the exact center of the waistband, lining up your chalk mark with the center back seam of the jeans.
STEP 4: Remove the stitching from the upper end of the center back belt loop. The lower end of the loop will usually be sewn to the center back seam of the jeans, and will not be affected by the following alteration.
NOTE Sometimes two other belt loops on either side of the center back will need to be released from the waistband as well. It depends on how much you are reducing the band, and how far apart the belt loops are positioned.
STEP 5: Remove the stitching that secures the waistband to the jeans, beginning about 4" out from one of the chalk marks and continuing to 4" on the other side of the second chalk mark. This seam is frequently sewn using a chain stitch, with the "chain" showing on the inside of the garment and plain stitches sewing on the outside of the waistband. To quickly unravel the chain, work on the inside of the waistband and cut through the stitching on both ends of your removal distance. With a needle or the tip of your seam ripper, catch the small thread tail inside the loop on the left hand side of the chained seam. Give it a tug, and the whole chain stitched seam will unravel to the point on the right side where you also cut through the thread. If the waistband is sewn on with a regular stitch, remove the section using a seam ripper.
STEP 6: Remove the stitching that holds the outer waistband to the inner waistband along the top edge. Almost always, a jeans waistband is in two sections. A lightweight denim may have a waistband that is simply folded over and top stitched, but the top stitching will still have to be removed to do this alteration.
STEP 7: Divide the distance of the alteration by four. In our sample, 3" divided by 4 = 3/4". You will have to reduce the rear of the jeans by the full amount (3") to fit the new, smaller waistband, but you will not do it in one place, you will instead make four tiny darts.
STEP 8: Cut the waistband vertically at your center back mark. Cut both the outer and inner waistbands at the same spot.
NOTE: You should now have two sections of back waistband flapping around loose from the jeans.
STEP 9: The next step involves making tiny darts in the back of your jeans below the waistband. The top end of each of these darts is going to be the measurement we found in Step 7...three-quarters of an inch for our example. These darts will reduce the waistline in the jeans to match the smaller waistband.
These darts need to be symmetrically located on either side of the center back seam. However, because the center back seam is a flat-felled seam, it is often difficult to accurately ascertain the true "center." I use the pockets as my guides instead of the center back. Beginning about 3" out from the center back seam, place a straight pin above the pocket, often even with the edge of the pocket that is closest to the center back. The pin is placed perpendicular to the raw waistband edge of the jeans. On the mirror side of the back, place a second pin above the OTHER pocket, at the same point. Next go over about two inches towards the side seam from these first pins, and insert two more pins, again making the left side and the right side of the jeans back look the same.
You should now have four, evenly-spaced pins sticking up from the raw edge of the back at the waist.
STEP 10: Using each pin as a guide, fold the waistline of the jeans at pin number 1. Stitch a dart at that point, sewing down to the yolk line below the waist. Each dart will be sewn one half the distance of the measurement in Step 7, which in our example is 3/8" (3/8"+ 3/8" = 3/4", which is the total amount of each of our four darts.) Continue to sew all four darts. Press all of the darts towards the center back.
STEP 11: Sew the outside of the waistband together at the center back cut, making the seam one half of the total waistline reduction (1.5" seam, using our example.) Reduce the bulk in the seam after it is sewn. Repeat this process on the inside waistband. You now have reduced both the inner and outer segments of the waistband the desired amount. Press the center back seams open.
STEP12: Sew the inside waistband on to the wrong side of your jeans, using a threat that matches the color of the jeans and lining up the seam with the secured section of the waist. Next sew the outside of the waistband to the right side of your jeans, again using matching thread. At this point the waistband should be back on the jeans.
NOTE: At this point, we are getting numerous layers of jeans fabric to sew. It's important that you use a #110 needle, or a "jeans" needle, here.
STEP13: Next, sew top stitching to match the manufacturer's stitching on the waistband. (See the blog on hemming jeans for thread tips.) Use a heavy duty thread as your top thread, and continue to use the color matching the jeans for your bobbin thread. Tighten the upper tension, and lengthen the stitch length setting. Beginning at the point where the original top stitching ends, begin to sew, back tacking at the start and finish of the seam. You will likely have to do this on both the lower edge of the waistband seam and the upper edge.
STEP 14: At this point, the only fabric not secured should be the top of the belt loop at the center back. Keeping the top stitch thread in the machine, secure the belt loop to the altered waistband. (If you have loosened additional belt loops, secure them as well.)
Yea! You did it! You, the lucky gal with the tiny waist! Way to go!