We can set up to 3 custom close reasons. Do we want any custom close reasons?
This can be tricky; your goal here is to communicate clearly with the people whose questions are being closed, with the people who are closing questions and with the people who will later read those questions and wonder why they were closed.
At a bare minimum, off-topic reasons should identify a specific topic considered inappropriate. If you're struggling to be specific, find an example of a question that is off-topic and discuss the factors that make it inappropriate for the site.
Whenever possible, try to explain why a given topic is not allowed - this is your chance to answer the inevitable question once rather than repeating it every time a question must be closed.
Provide resources that will aid askers in solving their problems: either instructions for asking the question in a more suitable fashion, links to a different site where the question may be considered on-topic.
Be as concise as possible. Save lengthy explanations and examples for meta FAQs that can be discussed and refined over time, and then include links to these in the off-topic reasons. Don't depend on anyone following these links though; keep critical information and guidance in the reason text itself.
Do all of this on your meta site so that folks can critique the wording and suggest improvements.