Answer:
Lets say that you're bisecting segment AB.
Explanation:
1. Place the compass on point A, and open it wider than half the distance of the segment.
2. Swing an arc above and below the segment.
3. Keeping the compass the same width, place the compass on point B, and swing an arc above and below the segment.
4. Place points C and D on the intersection of the arcs created above and below segment AB.
5. Draw a line from point C to point D.