There are many essays on Wikipedia about desired and poor behaviour (see Wikipedia:Essays in a nutshell/Civility and Template:Civility for links). Although they are useful as link destinations, expanding on concepts being referred to in conversation, newcomers coming across these essays may miss the forest for the trees: the essays provide a mass of specific details without grounding them in a framework of guiding behavioural principles. Describing a baseline set of principles for expected behaviour is useful, particularly with an editing population that spans many different cultures.
Phase 1: collection of scenarios edit
Gather examples of desired behaviour and poor behaviour.
Give everyone the chance to submit one scenario. It should be concise and brief (for example, no more than five sentences), so no one submission dominates the others. After a period of time, let everyone submit another scenario. Repeat as needed.
Phase 2: identify common themes edit
Based on the existing set of related essays and the scenarios collected in phase 1, look for common themes.