To bisect a line AB, or cut it in half, we need to draw a line through the middle of the line AB, perpendicular to AB.
We use a compass. Put the needle at A, open the compass to more that half the distance of the line AB and draw an arc through the line.
Now put the compass needle at B, without changing the compass distance, and draw an arc through the line. Make sure that enough of an arc is drawn so that it crosses the first arc.
Between the points where the arcs cross draw a line. This line will bisect the line AB.