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.
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).