In his manual of zen training Selling water by the river,
Jiyu Kennett says there is a special small ceremony before using
the lavatory; this verse should be recited three times:
Adoration to all the Buddhas.
Adoration to all the limitless teaching.
Peace! Speak! Blaze! Up! Open!
To the glorious, peaceful one for whom there is no disaster whilst upon the water-closet, Hail!
Dogen Zenji's Shobogenzo devotes a whole chapter to Senjo. Those practising outdoors must use a privy, and for cleaning theselves arrange seven balls of sand into two rows. Monks practising indoors must enter the lavatory, or tosu from the left-hand side of the corridor, with a towel folded in two on the left shoulder, and purify the lavatory by snapping together three times the thumb and index finger of the right hand. When finished hold the water bucker in the right hand, cup water in the left, and wash the genitals and buttocks three times.
(See them on the Web.)