Long diagonal is ridden M-X-K, or K-X-M or H-X-F or F-X-H. A short diagonal is ridden H-B, B-K, F-E, E-M.
When I first started to ride I was told to change vertical over X, however my now trainer has me change just before I hit to next corner, if I'm left changing to right rein, the bend stays left. And vice verca.
One super excercise I have been taught it change the sort diagonal, and once you hit E/B, change the diagonal through the short side again, so it makes a pyramid type top shape, if that explains it haha!