See, for sketching the mid point of a segment u need to follow these steps :

1) draw a line segment CV (of specified length , if given), otherwise draw a line segment of any length.

2)putting the spike end of the compass on one end of this segment, draw  2 Arcs,one above the segment and 1 below the segment. 

3)repeat step 2 from the 2nd end of segment.

4).arcs drawn from both the ends will cut each other, on above as well as below the segment.

5)Match these two points with a Scale, so, the line drawn by matching these intersection points will cut our main segment CV. say at point p.

6) this point p is the mid-point of segment  CV.