Disconnect the PS3 line. Position spray manifold in the inlet cowl, position collector unit behind the engine. Dry motor the engine, turn the supply to the spray manifold on. Allow to dry motor, then switch the water supply off to the spray manifold. Let engine wind down, collect water sample to measure PPM (cannot recall exactly what contaminant). Repeat another couple of times, measured PPM will drop each time.
LEAP-1A: try to wash every 750 FC. Need to stick to this for the higher thrust rating engines.
CFM56-5B: wash annually. When fuel was cheap, did not bother on the A319's.