The key questions to be decided are
what its purpose is;
where it should be used;
how it should work;
what it should cover, what it should link to;
how it should change its appearance, etc.
It's also possible to have a set of Helpboxes, from which users can choose, e.g., on their Gadget Preferences. Ideas are welcome!
The placement of certain elements, such as external links, fonts and lists, may be governed by subtle rules. Some of these we can program as constraints or warning windows into the script, e.g., warning the editor that they're about to add an external link into a section not labeled "External links".