A set square is a plastic triangle. You can usually buy them in a set of measuring utensils that includes a ruler as well. Set squares come in different forms. You may have one that is skewed to provide a 30°, 60° and 90° combination, or it may be symmetrical and contain a 90° angle and two 45° angles. You can calculate an angle with a set square, but it is better to use a protractor, which shows a wider range of angles.
Place the paper depicting the angle you want to measure on a flat surface. The angle exists between two lines. Place one edge of the set square along one of the lines. Flip the set square over to try a different angle. Try the 30°, 45°, 60° and 90° angles.
Note if one of the angles of the set square matches the angle being measured exactly. If so, this gives you your angle and you do not need to do any more work.
Apply an adjustable set square to the angle. Place the shorter side of the adjustable set square to one of the lines that make the angle. Unscrew the locking nut on the set square and open up the adjustable side until you match the other line that makes up the angle. Read the angle degree off the sliding scale on the set square.