Place fabric right side together (RST). Pin sides and bottom and sew down.
In the bottom right corner, measure 2" away from the side and the bottom seam and trace a square.
Pinching the corners of this square, match the bottom seam with the side seams. Press flat, arrange the seams so that one points to the right and the other to the left. Pin and sew down.
Then either fold the flap down to give extra body and sturdiness to the bottom of the bag.
Or simply trim it off.
Turn bag right side out and push the corners out.
Method #2
Place fabric right side together (RST). Pin sides and bottom and sew down.
In the bottom right corner, measure 2" away from the side seam and the bottom seam and trace a square. Using your scissors, cut out the square.
Match bottom and side seams, pin and stitch down with desired seam allowance (1/4" should be fine).
Turn bag right side out and push the corners out.
Method #3
You will need a single strip of fabric for this method. Fold rectangle in half, right side out, and press.
Measure 1 1/2" away from the fold, mark and place a ruler over the fabric.
Then fold one side over the ruler.
Press using a pressing iron. At this point the rectangle should be wrong side out.
Turn it right side up and repeat the steps for the other side of the rectangle.
Now the whole rectangle should be wrong side out.
Place fabric so the fold is at the bottom and raw edges at the top.