If you note that a square can be circumscribed by a circle (circumscribed is when the object is drawn in...right?) this makes things handy.
Draw a circle with the compass, for the diameter = side length of the square.
Drawing the circle ensures that the side lengths will all be congruent.
Now take your straightedge, and draw 4 sides, 2 sides parallel to each other on 2 outer sides of the circle, and 2 on the top, also parallel, BUT also perpendicular to the other 2 lines. You have a square
