Start off by centering on of the side seams so it lines up with the bottom, center fold. Pinch it together and lay it on your work surface. Use pins to hold it together.
Use a ruler and measure up 1 1/2" from the point of the fabric and draw a line. The line should measure 3" long with 1 1/2" of fabric on either side of the seam.
Repeat this for the other side.