Hemming jeans is a basic life skill, and if you own a decent sewing machine, you can do it!
SETTING THE HEMS
Step 1: Prepare the jeans. Never pin up a pair of washable jeans, pants, or trousers unless you have first washed and dried them. This very first wash/dry will cause a reasonable amount of shrinkage, which means that when you shorten them with the new hem, you have an expectation that they won't shrink much more.
Step 2: Find a helpful friend. You need one person to wear the jeans and the second person to set the hem. Trust me, you cannot accurately set your own hem.
Step 3: (duh!) Put on the jeans.
Step 4: If you are a belt-wearer, put on the belt. Depending on your body type, using a belt can lift the pants legs 1-2" in some instances. Since we're going for the perfect finished length, we want to minimize miscalculations.
Step 5: Put on the shoes. Which shoes? The shoes most likely to be worn with the jeans. No, ladies, it is NOT possible to have one pair of jeans that looks great with both flats and heels. Hem one pair for going out on the town in heels, and a different pair to wear with boots or flats.
Step 6: Set the hem. Most of the time, this is done at the back of the heel. I turn the pant leg under and line up the folded edge with a) the line between the heel of the shoe and the upper of the shoe, b) the floor (for those who like really long jeans, or c) pick some other agreed-upon distance up from the floor. Pin one leg...then pin the other leg. Injection mold dolls are the same on both sides of their bodies, but people aren't, so don't succumb to the temptation to just pin up one leg. Setting the hem at the back will usually create a slight break in the pant leg at the front of the foot, allowing a bit of excess length to cover socks when the wearer is sitting down.
Exception: If you want the pant leg to fall straight down the front to the top of the shoe or boot, and to form no break, then pin the pants at the front of the foot. This is often preferred by shorter people of both genders, who object to any excess fabric around the foot.
PREPARING THE JEANS AFTER SETTING THE HEM.
Step 1: Take off the jeans (duh!)
Step 2: Place a pin right on the fold line that your formed when you set the hem. Measure up the distance from the original hemmed edge to that pin you placed at the fold line, and place a pin through a single layer of the jeans leg. Do this about every 4" around the leg. Repeat on the second leg.
Step 3: Measure down 1.25" from each pin toward the lower edge to create the fabric amount that will actually become the new turned up hem. Use fabric marking chalk to mark 1.25" down from each of the pins you used to mark the whole circumference of each leg.
If you are only shortening the jeans a very little bit, you will have to remove the original jeans hem that was sewn by the manufacturer before you can get your 1.25" of hem length. Remove the hem stitching, and then press the folded hem flat with an iron set on "cotton" for standard denim, or use a slightly lower setting for stretch denims. Then mark as in Step 3 above.
Step 4: Cut off the bottom of the jeans along the chalk marks you made 1.25" below the pins.
SEWING THE JEANS HEM
Step 1: Put a #118 or 'jeans' needle into your sewing machine. The thickness of many denims is too much for the standard #80 or #90 needle. It is worth buying the right size needle.
Step 2: Thread the machine with a bobbin thread and a upper thread that are the exact color of the jeans you are working on.
Step 3: Set the machine to do a long stitch (6-8 stitches per inch.)
Step 4: Turn the jeans legs inside out.
Step 5: Fold the hem towards the inside of the jeans along the pin line. You can press each hem fold if you choose.
Step 6: Fold the lower edge of the fabric under once again, so that the raw edge barely hits the pinned fold line. You will now have a total of three layers of denim, and should see a narrow (5/8") folded hem.
Step 7: Start sewing the hem on the rear half of the jeans leg, just before the inseam. Beginning to sew at that point allows the back tacking/seam ending to occur at the spot least likely to be seen on the jeans.
Step 8: Sew all around the hem of each leg, back tacking to secure the seam at the starting and ending points.
The hardest part of hemming jeans is the thickness of the denim at the points where the hem crosses over the vertical seams of the pant legs. Here are two recommendations will help with theses areas.
First, purchase plastic "hump jumpers" or other plastic shim-type tools that will lift the presser foot even with the thickness of that bulky seam area. These go by a variety of names, and can be found in the notions department of any well-stocked sewing store or online supplier. I purchased mine from Clotilde.To use these lifts, sew the hem until you get about 3/4" away from a bulky vertical seam. Place the plastic lift/shim under the BACK of your presser foot, lifting it to a height even with the bulk of the seam, and slowly sew forward. As the presser foot begins to clear the bulk of the seam, move the lift/shim so that it is placed at the front on the presser foot as it moves away from the bulky seam, again keeping the presser foot at the same height as the bulky seam.. As soon as the presser foot has completely cleared the seam bulk, remove the lift/shim and sew as usual.
Secondly, I recommend that you hand turn the average home sewing machine across these bulky seam areas, rather than trying to power through with the machine. If you go slowly, the stitches are more likely to form properly.
Happily, the vertical seam bulk on many fashion jeans is not as much of an issue as it is on true Levi brands or other heavy-weight jeans. Many fashion jeans hems can be hemmed without the help of the lift.
FINAL COLORED TOP STITCHING
Once you have the jeans hems seamed with matching thread, the hem itself is secure and the jeans may be worn. However, if you want your jeans hem to have the same contrasting thread color that many manufacturers use on jeans, you still have one final step.
Step 1: Purchase special heavy-weight, 'Jeans' thread in the color of your choice. Be sure to take your jeans with you to the store, for there are many subtle variations in thread color that can make or break the 'authenticity' of your hem job. I prefer Gutterman Threads, and keep 15 different colors on hand to meet the discriminating standards of my clients. Surprisingly, I often use a thread color that looks 'brown' to the eye, but sews up as a dark gold on denim. Because these heavy cotton hem threads easily absorb the dark dyes from washing a dark load of clothing, the thread on your jeans do not stay a true, say 'gold' color, once the jeans are washed. Thus it is the slightly-over-dyed color that you want to match.
Step 2: Turn the jeans back to right side out. Because the heavy weight threads cannot be easily used in a bobbin on a domestic sewing machine, I use the heavy, colored thread only as the top thread.
Step 3: TIGHTEN the tension for the top thread only. Leave the bobbin thread (blue or black) as it was when you sewed the initial hem. When you look at the new stitch line on the underside of the hem, you should not see any of the colored thread showing. (You may need to test the particular tension levels of your machine to get this just right.)
Step 4: Top stitch around the jeans hem, right on top of the hem you sewed in the matching color thread. Use the lift/shim as described above, and once again start and stop on the rear pant leg, just before the inseam.
Step 5: Remember to return your upper thread tension to normal when you are finished with the colored top stitching, and to remove the jeans needle as well.