I made most of my spending money in college in the late 60's patching jeans, and I've been doing it ever since. You can extend the lives of jeans and/or create some art out of your old jeans by learning how to patch effectively.
OUTER PATCH FABRIC
The fabric you choose for your patching will depend on your intention. If sturdy wearability is the goal, use a denim-weight fabric. While you can certainly use new fabric (pre-wash it to remove shrinkage and sizing,) I prefer used denim. I keep a bag of denim scraps in my studio, some of them claimed from hemming jeans that were way too long and some of them salvaged from jeans that could no longer be worn. In the latter case, I recommend cutting away the back of the lower leg for salvaging fabric, for this area rarely gets very worn. It is important that the denim you use for patching be sturdy. After awhile, you'll find yourself with a scrap bag containing all different shades of denim, and you will be able to match the jeans you are mending.
You can also pick up used jeans at thrift stores and cut them up for patches. Locally, try the Bargain Boutique, which has recently reloacted to the west end of Winslow Way, or the Goodwill Store across from Costco in Silverdale. Also, you can swap scraps with your friends to increase the variety of denim colors you have.
If the goal for your patching job is more decorative, you can use any washable fabric, but do make sure it has been washed. For true patching (ie, there is a hole underneath) it is best to stick to a firmly-woven fabric, for thinner ones are likely to just wear through again.
Fusible web is a non-woven fabric-like product that is usually found near the interfacings at a fabric store. Fusible web is used to adhere two layers of fabric together and keep them from shifting from side to side and creating wrinkles. With heat (from an iron) and water (steam from the iron, plus a damp cloth,) fusible web turns into "glue" that temporarily adheres the patch to the jeans. Esthers, our local fabric store, carries several different kinds of fusible webs, including "Misty Fuse," "Steam-a-Seam," and "Heat and Bond." These products all come with printed directions included with the yardage, and they come in differing thicknesses. Consult the clerk to help you select the right product.
I recommend running test fusings while you are becoming familiar with these products.
THE ACTUAL PATCH
Once you've selected your fabric, you will need to decide whether to place the patch on top of the hole in the fabric or underneath it, because the recommended sewing process does differ. A top patch results in a cleaner, more finished look, while an underpatch falls more into the 'art form' area of patching, and will allow the fraying denim to show on the outside while actually covering the hole.
PATCHING ON TOP OF THE HOLE
Step 1: Cut the patch slightly larger than the worn or torn area you are covering. When sturdiness is the goal, you want the outer edges of the patch to be sewn into sturdy portions of the existing jeans. Do not sew a patch onto a worn area. When cutting the patch, you will want to pay attention to the grain of the fabric, or, if it is a stretch denim, to the direction of the stretch. You will want the grain of the patch material to run the same direction as the jeans fabric under the patch. You will want the stretch of the scrap to be the same as the stretch of the jeans fabric. In each case, this assures that the patch will work with the base garment.
DEFITINTION OF GRAIN
The grain of the fabric is the direction that the woven threads run. The lengthwise grain of the fabric yardage runs parallel to the selvedges of the fabric. In jeans, it will be a line running straight down the front or back on the leg right in the center of the leg. The lengthwise threads are the stronger of the two, and they are what make the pant leg hang straight. The crosswise grain of the fabric runs at a 90 degree angle to the lengthwise grain. In most fabrics, the crosswise threads of a fabric are thinner or weaker than the lengthwise threads.
Step 2: Get out your fusible web material. Lay your patch upside down, UNDER the fusbile web. Because the web is somewhat sheer, you will be able to see through it. Cut the fusible web just 1/8" smaller than your patch.
Step 3: Place your torn jeans on the ironing board right side out with only one layer of the jeans on top of the board and the hole facing up. If the jeans you are mending are smaller than the pointed end of your ironing board, you can use a sleeve board and slip the leg onto it. Iron the area around the hole flat.
Step 4: Place the fusible web patch on top of the jeans, and then place the fabric patch right on top of the fusbile web. You should now have a sandwich; lower level = jeans; second level = fusible web; third level = patch.
Step 5: Following the instructions for using the fusible web, place your iron down on top of your patch sandwhich and hold for a moment. Next, take a dampened pressing cloth (a clean hankie works great) and place it on top of the pile and press again, usually for 10 seconds. Now the patch should be firmly stuck to the jeans.
If you have a significant hole in the jeans, the residue from the fusible web could soak through onto your ironing board cover. To prevent this, use a scrap of fabric to 'soak up' the fusible web, and immediately after pressing, pull the scrap off so it doesn't stick to the underside of the jeans.
Step 6: With your machine set to perform a zig-zag stitch at the largest width your machine will do, but with the stitches VERY close together, zig-zag all around your patch, through the jeans and the patch fabric.
When the patch must be placed lower on the leg, around the knee for example, having a free-arm sewing machine can be really helpful. When working inside the leg, I usually start sewing at the upper left-hand corner of the patch, going horizontally across the patch and then down to the lower right hand corner of the patch. I end my stitching there, and then I go back to my initial starting point. This time I work vertically down the patch and then across the bottom of the patch, to meet up with my first zig-zag line of stitching.
ALTERNATIVE TO ZIG-ZAGGING
If you don't have a zig-zag machine, you can prepare the patch in a slightly different way before you fuse it to the jeans. Cut the patch at least 1/4" larger than the area to be covered all the way around the patch. Using an iron, fold under each side of the patch 1/4" and press. Then cut out your fusible web and adhere as above. When you sew the patch to the jeans, use a straight stitch rather than a zig-zag stitch.
PATCHING UNDERNEATH A HOLE
Step 1: Cut a patch exactly the size you want it to be
Step 2: Using your serger, serge all the raw edges of the patch. Press it flat
Step 3: Get out the fusible web. With the patch facing up, slide the patch under the fusible web. Cut the web just slightly smaller than the patch.
Step 4: Turn the jeans inside out and place them one layer thick on your ironing board or sleeve board. Flatten the area around the hole with your iron.
Step 5: Place the fusible web on top of the hole; place the patch on top of the fusible web, completely covering it (as explained above.)
Step 6: Set your iron directly down onto the patch and hold for a moment. Lift the iron, place a damp pressing cloth on top of the patch, and set the iron face back down onto it for about 10 seconds. The patch should now be cleanly attached to the underside of the jeans. (See above notes about using a scrap of fabric to catch any fusible material that could stick to your iron.)
Step 7: Using a bobbin that is exactly the same color as the jeans, and a top thread the color of the patch, sew with a straight stitch all around the perimeter of your patch, just inside the line of the serging. This secures the patch to the jeans without it showing much on the outside.
Step 8: Turn the jeans right side out. The patch material will show through the hole in your jeans, but the hole will be showing on the outside.
Step 9: Change the top thread on your machine to a color that exactly matches the jeans being patched.
Step 10: Zig-zag or straight stitch around the perimeter of the hole opening, securing the worn jeans fabric around the hole so that it cannot get caught on other objects.