There are two ways I do boxed bottom. What you do is turn your bag inside out and fold it in such way that the one side seam is right on top of the bottom seam. Be as precise as you can. This will form a corner. Place your ruler a few inches away from the corner making sure that the side seam is at 90 degrees with the ruler. How many inches depends on how wide you want your bag to be. Draw a line and stitch on the line. Repeat the same with the other corner. Turn your bag right side our and - voila, you will have perfect square corners. The other way I do it is virtually identical except I do not turn my bag inside out. When I am done boxing the corners, I flip the little triangles up the sides of the bag and secure them with decorative buttons I hope this helps. Good luck!