They have a picture of an arena there - centerline goes from A to C through X. (Rider enters at "A" for test, Judge sitss outside of the arena behind "C" as it helps them see how straight you move from A to C.)
Quarterlines are parallel to centerline, between centerline and long sides of the arena.
Diagnol lines can be Long diagnols are: H through X to F or M through X to K
Short diagnols are from long side to other long side just shorter distances than the long diagnol, for example: K through L to B, or V through X to R or V to M.