Cut your fabric pieces out of sturdy fabric, such as canvas. You can also use another type of heavy-duty fabric. To keep yourself organized, label the different pieces using a dressmaker's chalk or pen. Listed below are the pieces you need to cut and their measurements. You can make a bigger backpack if you want, but keep the proportions and seam allowances the same.
Front panel: 12½ by 14½-inch (31.75 by 36.83-centimeter) rectangle
Back panel: 12½ by 15½-inch (31.75 by 39.37-centimeter) rectangle
2 zipper panels: 3 by 17-inch (7.62 by 43.18 centimeter) strip
1 side panel: 4½ by 30-inch (11.43 by 76.2 centimeter) strip
2 long straps: 6½ by 25-inch (16.51 by 63.5-centimeter) strip
2 short straps: 4 by 6-inch (10.16 by 15.24-centimeter) rectangle
1 handle strap: 3 by 6-inch (7.62 by 15.24-centimeter) rectangle
Shape the top of your front and back panels. Pin the two panels together, so that the top, narrow edges align. The back panel will be a little longer. Fold the panels in half lengthwise, then cut the top into a large curve. When you unfold it, the top should be arched, just like a real backpack. Unpin the panels when you are done.
Shape the bottom corners of your front and back panels. Pin the two panels together again, but this time, align the bottom, straight edges. Fold the panels in half lengthwise, then cut the bottom corners at a curve. Don't make the curve as big as you did for the top. Unfold, and unpin the panels when you are done, and set them aside.
Use a glass and a dressmaker's chalk or pen to trace the curve.
Fold the straps. Take one of the long straps, and fold it in half lengthwise with the wrong sides facing together. Press it flat with an iron using a heat setting suitable for the fabric you are working with. Tuck the long edges inside, all the way to the crease. Press the strap flat again.
Do this for both of the shoulder straps, the short straps, and the handle strap.
Topstitch both of the long edges down. If you need to, pin the strap shut with sewing pins. Topstitch along both of the long edges using a thread color that matches your fabric.
Do this for the both of the shoulder straps, the short straps, and handle strap. Set the straps aside when you are done.
Cut the back panel into two parts. Find the top center of your back panel. Measure down 4 to 4½ inches (10.16 to 11.43 centimeters), then cut straight across. You will end up with two pieces: a curved top piece and a rectangular bottom piece. The top piece will be referred to as "Part A." The bottom piece will be referred to as "Part B."
Place the long straps on top of "Part B." Make sure that the right side of the fabric is facing you, then place the shoulder straps on top. They need to be about 1 to 2 inches (2.54 to cc centimeters) away from the side edges. Make sure that the top edges of the straps are aligned with the straight, top edge of Part B, and that you are pinning the straps to the right side of the fabric.
Pin Part A face down on top of the shoulder straps and Part B. Make sure that the bottom, straight edge of Part A is aligned with the top, straight edge of Part B. The wrong side of Part A should be facing you.
Sew Part A, Part B, and the straps together. Use a ½-inch (1.27-centimeter) seam allowance and a straight stitch. Remove the pins as you sew.
Flatten the back panel, then topstitch the seam down. Fold Part A up, so that you can see the entire back panel. Fold the seam down against Part B, and press it flat with an iron. Topstitch the seam down to Part B; you will be going right over the shoulder straps.
Assemble the bottom straps. Fold the short straps in half widthwise. Slide a backpack strap adjuster onto each strap, so that the bars are nestled in the folds.
Pin the bottom straps to the back panel, and sew them in place. Turn the back panel so that the right side of the fabric is facing you. Pin the bottom straps to the bottom of the back panel; make sure that the raw edges align. Also, make sure that the bottom straps align with the shoulder straps. Topstitch them in place using a ½-inch (1.27-centimeter) seam allowance.
Finish the shoulder straps. Fold the raw, bottom ends of the shoulder straps twice: first by ¼-inch (0.64-centimeter), then by ½-inch (1.27-centimeter). Topstitch the folded edge down. When you are done, thread each strap through the corresponding strap adjusted at the bottom of the bag.
Attach the handle to the top of the back panel. Find the center of the back panel, then pin the handle to the top. Make the raw edges of the handle align with the top edge of the panel. The ends of the handle need to be about 3 inches (7.62 centimeters) apart. Stitch the ends down using a ½-inch (1.27-centimeter) seam allowance.
Pin the two zipper panels together, then sew them together along one of the long edges. Pin the two zipper panels together with the right sides facing in. Sew along one of long edges using a ⅝-inch (0.59-centimeter) seam allowance.
Press seam open, then baste zipper on top of seam. Open the panels, so that the wrong side of the fabric is facing you, then use an iron to press the seam open. Baste the zipper on top of the seam. Make sure that wrong sides of the fabric and zipper are facing you; the right side of the zipper should be facing the seam.
It may seem like you won't be able to use the zipper because of the seam. Don't worry, you will open the seam later so that you can use the zipper.
Topstitch the zipper in place. Sew the left and right sides of the zipper down first, then the top and bottom. Be sure to sew across the top and bottom a few times so that it is nice and strong.
Open the seam on top of the zipper using a seam ripper. Turn the zipper panel so that the right side of the fabric is facing you. Find the top and bottom stitching for the zipper. Open the seam between those two spots using a seam ripper. You should now be able to open and close your zipper!
Be careful not to rip the top and bottom stitching that are holding the zipper in place.
Once you are done, cut off any excess zipper, if needed.
Sew the side panel to the zipper panel to make a loop. Pin the narrow ends of the side panel to the narrow ends of the zipper panel to make a loop; make sure that the right sides are facing each other. Sew the narrow ends shut using a ½-inch (1.27-centimeter) seam allowance.
If you'd like a neater finish: press the seams down against the side panel (away from the zipper panel), then topstitch it in place.
Pin the loop to the front of the bag, with the right sides together. To ensure that your bag comes out even: find the top and bottom center of your front. Mark it with a pen or pin. Find the center of your zipper panel and the center of your side panel, and make a mark as well. Align these marks while pinning the loop to the front panel.
When you are done, pin the back panel in place as well. Make sure that the wrong side of the back panel is facing you.
Sew the loop in place. Use a ½-inch (1.27-centimeter) seam allowance, and remove the pins as you go. To prevent bunching, cut small slits into the seam at the corns. Be careful not to cut into the stitching!
Snip off any loose threads, then turn your backpack inside out. Your bag is now ready to use!