well first off there really isn't one.. I did mine by wraping fabric around my waist to get the right amount of fabric. You want it to fall right around the middle of your calf right above your boots.
The Buttcape *as a few like to call it* is not just a regular square. If you cut it like that you get the v shape up front instead of the straight lines. The top part (around your waist) has to be shorter then the long part (bottom) and angle the sides to fit. This way the edges of the cape sits straight. Some people interface theirs to get the right look. Mine isn't interfaced because the fabric is heavy enough to get the right look (that and my big but)
I got my hints from a very good cosplayer AVAAntares (who has a tutitorial on the military uniforms military uniform tutorial
Mine is held shut by velcro (under the waist band) because I wanted to adjust it should I gain or lose weight. It's a separate piece from the pants.
Some use eye hooks or buttons for theres. I found velcro the easiest for me.
Hope this helps.
Should you need any other hints let me know. The military outfit was my first cosplay outfit I put together on my own. My other friend has made 5 jackets already for 4 other people.