A line from the vertex through the angle, This line divides the original angle into two equal angles. You can find this line by using a compass divider (or a piece of string). Swing from the vertex so you cross both rays of the angle at the same distance. Then use the divider or the string to swing an arc from each of ray intersection points with the same distance of your swing. These arcs will intersect. Connect the vertex to the arc intersection point. That line bisects the angle.