Measure each of the internal angles of your triangle with the protractor. Place the protractor's center onto the angle's vertex or corner, aligning the base line of the protractor with the initial leg of the angle. Record the number along the protractor's top where the other leg intersects, employing the ascending numbers for right-opening angles and the descending numbers for left-opening angles.
Select the largest angle measured with which to work. For an example, say that the angles measured in Step 1 are 40 degrees, 45 degrees and 95 degrees. Select the 95-degree angle.
Draw a perpendicular line from the vertex of the selected angle to the side opposite it with a set square, also known as a triangle. Align the set square's bottom edge on the side opposite the angle, and line up its side edge with the vertex of the angle. Draw a line along the side edge from the opposite side to the vertex. The perpendicular line will create two right angles and two right triangles.