How to box the corners of a bag
If you’re new to bag making, you might not be sure how to make one with a squared-off bottom. When I made my very first bags, they’d either be totally flat or I’d use separate panels for the sides and bottom. This way is easier and cleaner looking.
The pattern
Your basic pattern concept is two pieces to make up your bag. They can be whatever size you want as long as they are squares or rectangles. From the bottom two corners, you will cut out a square. How big this square is depends on how wide you want the bottom of your bag to be. Take into account seam allowances and the fact that there are two sides, so your square will be doubled when they are put together. For instance, if you want the bottom of your bag to be two inches wide, on each panel cut a square 1.5″ x 1.5″. This is 1″ (half your bottom) plus 1/2″ seam allowance.
The process
Here is the notch I was talking about above. It’s a square cut from either bottom corner. In this photo, I have sewn together the sides of the bag along the sides and bottom, but left the cut-out corners unsewn. This particular bag has a two-toned exterior, so there’s a seam in the middle. But you can just be using a solid piece of fabric if you want. For the rest of the tutorial on making the bag start to finish, see this post.
You’ll start to separate the two sides at the cutout and pinch them together the other way.
You’ll end up aligning the bottom seam and the side seam. It’s easiest if you open the seam allowance.
Make sure those side and bottom seams align at least an inch or so beyond the edge of the notch and pin to secure.
Sew a 1/2″ seam allowance across this flattened area that you created. That seam you see running along the center will be either your bottom seam or your side seam, depending on which way you flipped your bag to sew this part.
Now you have a nice, flat seam that has made your bag more 3-dimensional. Do this on both sides and turn it right side out.
This is what you get. Nice, neat and pretty. See how key it is to align the side and bottom seams?