your sewing machine.
trim both seams on both sides of the bag, and fold each side as a bias and sew to close each side. don’t forget to place the keys ring while you close the bias in one side of the bag.
with the help of the patchwork ruler, draw a line from one inch from the point, place a pin to hold the fabrics while you sew it on your sewing machine.
after you sew each boxed corner, you can trip the corner with the help of a pair of scissor, and fold the fabric as a bias and close it by hand.
this is how both boxed corner should look like 