diagonals
It's less about rules than about the practical role diagonals play in going from the drafting table to the loft floor. Let's say you have begun a lines drawing. You've drawn a few waterlines and have sketched in sections at stations 2, 5, and 8. It's time now to define a diagonal on the "body plan" (the bow/stern view looking at the sections). You might start at the intersection of DWL and the centerline, and you'll probably look at the bow section and the midsection that you've drawn and pick the easily defined and drawn angle (30 deg, 45 deg, etc) that is most nearly perpendicular to both sections.
That's as detailed as I'm going to get here, but you might check YACHT DESIGNING AND PLANNING by Howard Chapelle or other classic design/lofting texts for more information.