Giving your fabric bags or purses a boxed bottom is so easy and it will give them a professional finish. There are a few ways to do it - I find this way easier.
1. Sew your fabric pieces wrong side together.
2. Using a grid ruler measure a square in each bottom corner & cut. This measurement can be anything - they must be equal & square though. The bigger the square, the wider bottom but you will reduce height at the same time.
3. I run back over the ends of the cut seams so they don't run when you open them.
4. Pinch open the cut squares and match seams.
5. Press seam open
6. Sew across. Turn in the right way and press again.
The key here is even seam margains. If you they are not even then the seams won't match up perfectly.