A protractor is a half-disc with two scales of degrees printed on it. The trick is just lining it up properly — once you do, reading off the angle is easy.
Three steps
- Center the cross-hair of the protractor on the angle's vertex.
- Line up the baseline (the 0° line) with one arm of the angle.
- Read where the other arm crosses the curved scale. Use whichever scale (inner or outer) starts at 0°.
If the angle opens to the right, use the bottom-left 0°. If it opens to the left, use the bottom-right 0°. Pick the scale that starts at the arm you laid the baseline on.