The actual great-circle distance of any point on the map to be plotted is measured from a point on the equator at 45 degrees west longitude, and the same from 45 degrees east. These two distances are then reduced to the scale of the map. The mapping of that point is then the place on the map where the linear measures from the points representing (45 W, 0 N; 45 E, 0 N) are those reduced distances. There are, in general, two points that satisfy these conditions, so points north of the equator are plotted above the midline and points south of the equator are mapped in the bottom half of the projection.
How is the equator itself represented on the resulting map? Consider it the limiting case of non-equatorial points if you like--this might be helpful for part of the answer. If more than one shape results, specify the range of longitudes along the equator that produces each shape.