Fred,
Maybe just a simple answer, from a simple person ;-)
Raindrop or ladder pattern: Drill holes or otherwise cut grooves through layers of damascus that has already been folded the number of times you wish. Forge again to force the layers that have been exposed to the surface.
Twist: forge layers, twist bar, then forge flat again.
Mosaic. Basically patterns cut to shape then welded together, or welded into the face of a damascus bar. I've seen some mosaic work where the pattern goes clear through the blade, while others have a different pattern on the right and left hand side.
The basic idea with these patterns is that the folding and forging has already been done to create layers. After that you are only forging one more time, but not folding anything....just heating it enough to pound the bar flat and smooth.
madpoet