You put a inch or two in the bottom and fill the rest with warm water,if you can use a hot tap to feed the pw you get a much thicker foam (you dont need to)
I have a two foam lances,one has snowfoam the other has a mild shampoo ,i foam with the snow foam let it dwell for 5 mins then rinse off,then i use the other lance and cover the car in shampoo (not as thick as foam tho) then i have a rinse bucket to rinse the mitt after a pass,then just pw off with water