Here’s another sewing tip that I had to learn the hard way. Boxing corners looks easy enough on paper, but when it comes to actually making those corners square, eyeballing it hasn’t been good enough for me. So for a tote bag that you’ve made by folding a rectangle in half and sewing up the sides, give that bag some depth by boxing the corners. And prevent those corners from being wonky by measuring carefully.
First, snip the seam open at the top and lay the side seam down the center of the bag. Press the seam open. Next, make sure the seam is truly centered by laying a ruler on the seam. Place a line on the ruler directly on top of the seam line, and as many inches down as you are supposed to box the corner. In this case, it is 3 inches (which is a very deep box as far as bags go).
Make sure that the fabric on the left side of the seam (A) is exactly the same as the fabric on the left side of the seam (B). If A does not = B, then adjust the box and measure again.
Now draw a line across the bottom of the ruler and pin fabric into place.
This is a bag with an ~1 1/2″ box.
Happy corner boxing!