Step 3: Cut the Fabric Pieces
Use the template to cut two pieces each from the lining and shell. To make your pieces bilaterally symmetrical, fold the template in half and place the crease on the fold of the fabric. Remember, you can fold the fabric wherever you like to make it most economical. Trace and cut. Ideally you'd use a pencil or chalk, but if you need to you can use a pen since the marks will be in the seam allowance. Disregard the added seam allowance marks in the pictures; I was foolish and didn't include them in my template.
Out of the pieces you've cut, one each of the shell and lining pieces will be the "back" pieces (whole rectangles, touching your back), and one each will be the "front" pieces (trianglish shapes, not touching your back). Using your template as a guide, cut one each of the shell and lining pieces along the diagonal that is marked on the template. You can use the first as a template for the second to get a perfect match. If your fabric has a right and wrong side, you will need to pay attention to which way the diagonals on the front pieces run. With your fabric right side up, lay the template wrong side up for the lining and right side up for the shell.
Finally, to make your bag hold a lot of stuff, you will need a rectangle of fabric to give your bag depth. Measure around a front piece from corner to corner the long way (not across the diagonal); see the last picture for clarification. Add an inch for wiggle room, and that is the length of the rectangle. The width is four inches (three inches of depth plus seam allowance), though you can make it wider or narrower if you like. Cut one each of these rectangles from shell and lining material.