Final answer:
The first step in constructing a perpendicular bisector for segment MN is to use a compass to draw arcs from both endpoints, which should cross above and below the line segment.
Step-by-step explanation:
The first step in the standard construction of the perpendicular bisector of a line segment, such as MN, is to place the compass at one end of the line segment (point M) and draw an arc above and below the line.
The compass width should be set to more than half the length of MN to ensure that the arcs cross above and below the line.
Repeat this process at the other end of the line segment (point N) with the same compass width, drawing two more arcs that intersect with the first two.
This is how you begin the construction of a perpendicular bisector.