Given ab, explain how to construct a square with sides of length ab

AB is a line, right?
To make a square, you have a quadrilateral with 4 right angles and 4 equal side lengths.
So, given AB, make two lines, make them perpendicular to AB, and the same length as AB. You should have a "U" shape of sorts, although it should have 90º where the lines intersect. Then, put one more AB-length line perpendicular to the two lines, so that you have a square.