STEP 1: Cut out two right triangles of fabric.
You want to cut out two equal sized right triangles of fabric. The larger your triangles, the larger your bag will be. My triangle edges measured 23 in. x 23 in. x 32.5 in., but I would have gone a bit larger if my fabric had permitted (bed ticking only measures 16.25 in. across). If you want the bag to measure a certain length across the bottom, you can employ a little math to help you work out the dimensions like so:
STEP 2: Sew the edges of each fabric piece to prevent fraying.
If you want to be fancy (and you are using a soft fabric) fold the edge of the fabric over (and iron) and then sew a nice hem all the way around each triangle. Or if you don't mind a raw edge like me, you can just go ahead and sew a line of thread about 1/8 of an inch in from the edge of the fabric all the way around to prevent fraying. I've emphasized where I put the thread line with a white dashed line in the photo.
STEP 3: Sew the triangles together.
And now we start the construction of the bag. Place one triangle on top of the other in a mirrored fashion, with the center of the triangles overlapping one another (as pictured). Attach the triangles by sewing a straight line down the center. Then, continue attaching the triangles by sewing down all the edges where the two pieces of fabric overlap. There should be no pockets. The two pieces will now function as one piece.
STEP 4: Sew up the sides, making sure to cut the bottom corners into angles.
This is where the bag is going to take its final shape. NOTE: I used a french seam as I sewed the bag into its final shape to keep a nice looking edge on the inside of the bag.
First, fold the large piece of sewn-together triangles in half (flip the bottom portion up over the top portion at the center seam, essentially) so you have the basic bag shape we will be creating. Make sure to match up the top tips of the fabric.
Second, pin if desired to keep everything in place while sewing.
Third, measure and mark approximately 3.25 inches up from the bottom corners and 3.25 inches in from the bottom corners to mark the edges of the angled corners on your finished bag. You can make the corners at whatever angle you want though, so feel free to adjust as you like.
Fourth, you are now going to sew up the sides of the bag. On the right side of the fabric, starting from one of the marks on the bottom of the bag, sew a straight line directly to the corresponding side mark on the same side of the bag. Then turn your fabric 45 degrees and sew straight up the edge of the bag through the upper tip of the fabric (sew your line off the fabric to complete the seam).
Fifth, repeat step four on the other side of the bag.
At this point you will have the bag in its basic form, but it will have two little floppy triangles on the bottom edges that you don't need; trim off that fabric and complete the french seam by flipping the bag inside out, ironing and sewing just on the outside the seams you made on the right side of the fabric.
Once you've finished the French seam on the wrong side of the fabric, turn the bento right side out, and admire that fine bag you just made! You're done. Enjoy your bento tote.