Ineed

explain how to construct a perpendicular bisector

So the first step is to have a line segment.
Put your compass on one of the points and extend the compass to be slightly more than half. Just a bit over half.

Mark a small curvy line above your segment and on the bottom at that length. 

Now pick up the compass, don't change the length of extension, and put it on the other point of your segment. 

Mark above and below your segment like before so it can intersect.

draw a line through the two intersections.

I hope I answered your question.