A diagional scarf involves planeing the upper side of one section and the lower side of the other section to a matching width and angle. Epoxy is applied to each cut scarf, the sections aligned and then clamped together. The joint is clamped and dried. A layer of wax paper is added on both sides if the joint during glueing to act as a 'release'. Both methods are easy to understand, but the glass-butt scarf is by far the easiest and fastest.
Diagional scarfs tend to deflect the woods less, and render smoother curves.
When scarfing dimentional timbers, use the diagonal scarfing method. The scarfs can be cut on a table saw, or a number of timbers may be clamped together and the scarf cut with a plane.