To measure the angle you have to draw an arc across the original.
Practice makes perfect
Let's first draw a straight line using a straightedge.
To copy the angle, we draw two identical arcs like below. Make sure the top arc crosses the two rays making up the angle we want to copy.
By measuring the angle on the original and then, using the same compass setting, measuring that same angle measure on the bottom arc, we can copy the angle.
Finally, we draw a line from A through C to copy the angle.
