Draw the base of the triangle, running your pencil along the straightedge. Then, turn your straightedge to a different angle and follow it in either direction. When you stop this second line segment, line up your straightedge between the beginning of your first line segment and the end of your second line segment, and draw a line segment to complete the triangle.
Place the protractor so that the bottom of the inside semicircle runs along one of the line segments in your triangle. Move the circle in the center so that it contains one of the angles centered inside it. The second line segment should run into the rounded part of the semicircle. The number or line on the protractor closest to your line segment indicates the number of degrees in that angle. If the angle is obtuse (larger than a right angle), use the numbers greater than 90 degrees. If it is acute (less than a right angle), use the numbers less than 90 degrees.
Write the angle measurement inside the angle you just measured. Then, move to the second and third angles, recording each measurement inside the triangle. If you have been using your protractor correctly, the angles should add up to 180 degrees.