Draw a long line up the middle at 90 degrees (i.e. square) to the first line.
Take your ruler and put the zero mark on the left end of the baseline. Swing the ruler over and line up the 5" mark with the vertical line. Draw this side of the triangle.
Draw a line from that point to the other end of the baseline and voila, you have an equilateral triangle. Incidentally these diagrams were created on Touchdraw, a £6 iPad app (apparently it will soon also be available for macs) which I am head over heels in love with because it is making me look so fancy and professional. Expect many diagrams on this blog from now on.
Instructions on making the copa cabana triangles will follow in a day or two once you have had a chance to get hold of some freezer paper and make some triangles. You can make your three triangle templates from template plastic or ordinary paper but I like freezer paper the best since you can iron it onto the fabric, cut and then peel away. Once you have made your three triangles, take your ruler and draw pencil lines on the non-sticky side of the freezer paper 1/4" from the edge of each side of the triangle. I will explain why in the next tutorial.