If you don't have a carpenter's square, you can still ensure that you are scribing a right angle by using the principles of mathematics. The formula for a right triangle is this: a^2 + b^2 = c^2, where "^2" means "squared." Consequently, if you measure the two sides of your angle, you can use the "c" factor  also known as the hypotenuse  to make sure that your right angle is, indeed, square. So, you see: Paying attention in high school geometry class really can pay dividends in real life.
Things You'll Need
 Tape measure
 Pencil

Using a tape measure, measure 3 inches on one side of the angle you want to be at 90 degrees and make a mark with your pencil. Then measure 4 inches on the other side of the angle and make a mark.

Measure between the two marks and you should have 5 inches. If you don't have 5inches between the marks, move the sides of the angle closer together or further apart until the space between measures 5 inches.

Just remember that 3 + 4 = 5. (Or, in our equation, 3^2 + 4^2 = 5^2.) This will work for a right angle every time. If your project is large you can use three feet on one side and four feet on the other side (instead of inches).