I've been wanting a little backpack, something just large enough to carry shopping bags to the store or hold a notebook and lunch. Sometimes I'll use a normal tote bag, but it has to be the right size, the straps long enough but not too long and so on. A little decoration would be interesting, but not so much bling that it matches the chrome on my truck. So why not just make one?
I had some old jeans hanging around that were no longer wearable but not entirely destroyed. This particular pair had some nice seam detail on the inseam, so I picked apart the outside leg seam and cut off the waistband to get a mostly flat section of fabric with a finished seam running down the middle. It will never be entirely flat because of the shape of the crotch seam, so I designed a bag with an elastic opening at the top so it wouldn't be a problem. I also took off one of the back pockets to use later.
To get a more backpack style while keeping a nice overall rectangular shape, instead of just cutting a rectangle I made one side narrower and the other wider. I could have just drawn a trapezoid directly on my fabric, but I made a paper pattern to better illustrate how to draft the correct shape.
Below is a picture of my paper pattern pinned to the ironed pant leg. You can see the crotch on the left and the leg hem on the right. Running down the center, under the paper, is the original inseam with its nicely topstitched finish. The crotch is bunched up under the pattern, but as long as you cut that edge in a straight line it doesn't matter because there will eventually be elastic there.
I've marked some lines on the paper pattern. Along the leg, there is a center line that runs from the top front to the top back. Halfway it is crossed by a line marking the bottom of the bag, 30 cm wide. The narrow end is 12 cm. To keep the same overall rectangular shape, the wide end is 30 cm (the bottom width) plus the difference that was taken away at the narrow end (9 cm on each side) for a total of 48 cm. Or you could just draw a line connecting the middle and narrow edge points and extend it to the wide side to get the same thing. All that really means is you are taking some away from one side and adding it to the other. The result is still a rectangular bag, but with the seams shifted at an angle on the back. You can make it as large as you can fit on your fabric. After accounting for seam allowances and a casing for my 3/4 inch elastic, my measurements gives a bag that nicely holds a standard composition book (the no-expense-account Engineering Notebook!)
Since I had more fabric left, I cut the shoulder straps along the outside edge of the fabric. On these pants, it is nearly straight grain, a good thing to make non-stretchy shoulder straps. Cut them four times your desired finished width and as long as you want them plus two seam allowances. Mine are about 70 cm.
Fold the straps in half lengthwise and press to set a crease. Open it up again, fold and press both edges into the center and then fold it along the first crease. As you can see in the photo, there's a lot of pressing going on here, and it's required to get a nice flat and straight strap. Stitch along both long edges, being sure to catch all layers.
Now you can embellish the bag and the straps any way you like. I added some decorative stitching to the back pocket and attached it to the center of my bag and then did some matching stitching on the straps. You can do basically anything here, just remember that the center of the wide end is what gets seen when you wear the finished bag, and some of the top edge will be folded over into a casing. A tip: if the edge stitching on your straps isn't so great, decorative stitching down the center will hold it together even if your first attempt didn't. You can also use decorative stitches to reinforce worn places or apply bits of fabric or ribbon to cover holes in the denim.
Now to assemble the bag. Fold along the bottom (the middle line from the pattern,) pin the straps in place (two ends of one strap in each seam) and match up the edges. Make sure the top end of the strap is far enough down to leave room for the elastic casing. (Mine is 9 cm from the top edge.) Stitch using a 1.5 cm seam allowance and then overcast the seam allowances together. I did the seam, then a stitch and overcast along the edge and another overcast only stitch over it again. You don't want this thing coming apart, and those straps are thick. While I was at it, I overcast the top edge so I don't need to fold under a hem for the casing.
Mark and stitch the casing based on the width of your elastic, leaving an opening to insert the elastic in the back. I made the casing wider than I would normally because the thick seams are hard to get the elastic past. I cut my elastic the length of the back top edge plus half the front top, but you may want less.
Insert the elastic into the casing, overlap and stitch the ends together to join and stitch the opening closed. I left the elastic unstretched in the back portion and then stitched over it along the seams to hold it in place. The elastic in the casing gives a nice stable edge between the shoulder straps, with only the front part of the bag opening gathered.
Turn the bag right side out and it's ready to wear.