Elbow grease mostly, but when you feel you've done enough, screening.

If you need consistent particle size with a mortar and pestle, you regularly screen whatever you're grinding with a tamis.

Eye hand coordination

Presumably you're joking, but even assuming perfect articulation of a mortar and pestle, the inherent uncertainty with how coffee breaks when cracked is going to lead to inconsistent particle size. The reason burrs actually work for precision is due to the fact that they cut the coffee until it is small enough to exit the burr gap. There's no such restriction on when coffee is finished cutting on a mortar/pestle.