The below pictures and instructions
will provide a clue of how these Megaliths were built. This is of
course, the Type A Flat Circle plan. Remember that Professor
Thom found that he could use the Megalithical
Yard to construct these plans.

To begin, we construct a horizontal line with two other lines forming 30 degrees with the horizontal line as shown above. These two other lines, shown as blue, are equal in length.

We can use those two blue lines as the green arc's radius to contruct the green arc.

From point A, which marks the lower quadrant of the green
arc, we construct the line AD as shown. Line AD meets OC at
the midpoint of line OC. Hence, OD and DC are of equal length.
We repeat this for line OB, shown in the picture below.

We then extend the two lines AE and AD respectively, so that the extended
portion, shown in green, is of equal length
to EB and DC respectively.

Again, we use the lines EB and DC to construct the arcs BM and CN respectively,
as shown. Also, lines AM and AN are the radii of arc MN. We
can use them to construct this arc.