The hardest “plug holes”  are the odd shaped ones.  laid flush (so you cannot feel them)

The hardest “plug holes”  are the odd shaped ones.  laid flush (so you cannot feel them)

Step one (see below). Find a sheet of paper, for this demonstration I am using an A3 sized sheet of plain paper.  I have cut a hole and laid it over the drain plug

Next, I have marked the paper using a straight edge