After a less than satisfactory experience last weekend with a seasick landlubber I think simple labelling is essential. My system is a 20 year old pump out with not very intuitive graphics to show the switch position for "flush" and "empty". I have an instruction sheet similar to Weyalan's but with a little less detail - and separated the plumbing
instructions on the reverse.
The first mate had used the head
once or twice before, but was virtually incapacitated by seasickness, and could not get it right this time. Her glasses were in the cockpit
so she could not read the instructions. She called up to me to say she could not empty the bowl, so I offered to come down and sort it out, which was declined due to it being "a bit messy".
Little did I know at the time that the bowl was actually full. With the lumpy seas I probably need not go into further detail.
So, in hindsight, I will revise my instructions - using large enough arrows to show FLUSH and EMPTY settings so they can be read without glasses.