You should have the reflector and driven elements installed on the boom.  The director elements are cut to length as called out in the layout page.  Mark the center of each element and then mark the distance on either side of center half the thickness of the boom.  This will give you a way to center the element in the boom.

The boom is drilled as called out in the layout page with a drill the same size as the elements.  Slide the element through the boom, center the element and then place a drop of epoxy on both sides.

Now it is time to install your boom to mast clamp and get it up in the air.