Alison should start by putting the point of the compass on the extreme M (or N) and draw an arc with the compass opened more than 1/2 of MN and less then the whole MN length. This should be her first step.
She should then do the same, but now placing the point of the compass on the other extreme (extreme N or M, depending on which one she started by).
She should now repeat the process but to the other side of the line. After all she then traces a line that passes both crossings of the arcs.