If you have enough rocks in the area, building with less batter than in the diagram makes things easier.
Also, if you are doing multiple tiers of rocks, the keystones really only need to be dug in enough so that they can not roll out downhill and away from the trail. The weight of additional tiers of rocks will hold them from shifting in other directions. If you only have one tier of rocks before the riding surface, then they had better be solidly dug in and shimmed together.
One thing which is probably pretty obvious is that when working on a slope, you always want the heavy/large end of the rocks uphill as this is inherently more stable as the rocks mainly want to roll downhill of course. As Joe Dick said, oblong rocks oriented with their long side parallel to the slope are better than spherical rocks.
But it does depend on how much rock you have, what shape the rocks are, and the slope of the area. By the time you build up to the trail bed and filling in with gold, if nothing budges with kicking or jumping on the rocks then you are probably good to go. If things move around when you kick the rocks closest to the trail bed then you probably want to beef it up before filling in the trail bed.
From what I have read, and my experience, a minimum good cover is 4" of mineral soil, and as Jerry said, more is better if you can.