Describe a process you would use to create the perpendicular bisector to a segment ab using only an unmarked straightedge and an unmarked compass.

You'll have to compass tip on A and draw a small ark with pencil approximately in the middle above AB line, now compass tip to point B and cross the ark you made previously.
Do the same on the opposite side without making any change to the compass 
Join the lines where crosses of arks on the both side meet and then ,it's done.