You know when you want to give your bag/purse/pouch a bit of depth at the bottom, you have to make that seam across the corner? And you know what it's like trying to fiddle around matching up the seams so it looks neat, sticking a pin through, wiggling it about a bit, making sure there's not a pleated-up bit on the other side.....well now you don't need to do all that. Here is my way of making boxed corners.
1) Sew your side and bottom seams as normal. If you're leaving a gap in the bottom seam for turning, make sure you've left enough room for the box corner seam.
2) Mark a square on each corner that you want to box. To work out the size of this square, first calculate how long you want the finished box seam to be - let's say you want an 8cm seam. This will be split between the two sides of the bag, so that's 4cm each side. Then take off your seam allowance, say 1cm, and you're left with the square size - in this case 3cm. See the diagram below! Mark this accurately (e.g. with a grid ruler) using a fading or washable marker or a pencil.
3) Get your sharp scissors out. Making sure your sewn piece is lying nice and flat, chop out this marked square - eek! A hole in your bag!!
4) Now pull the sides of the bag apart, so the seams meet (or seam and centre bottom fold in my case). Finger press seams open, check they match a little further down by peeking in, pin in place, then finger press the 45 degree fold at each edge.
5) Stitch across the raw edges you just cut and pinned, using 1cm seam. Your corner is perfectly boxed!