For templates, insert <noinclude>{{subst:doc}}</noinclude> at the end of the template, then save the page, this will place {{documentation}} and a comment advising editors where to place categories and interwiki links. For other types of pages, the documentation template should already be there.
Click "create" at the top of the page and the documentation page will be opened with some pre-formatting. This formatting can also be created by using {{subst:doc}} on the documentation page.
Documentation pages will automatically be added to Category:Documentation pages. Using {{doc}} without subst: will add the page to Category:Pages with templates requiring substitution.
Optional parameters
This parameters can either be added to the {{subst:doc}} template when creating the page, or added to the {{documentation}} template afterwards.
type
Sets the "type" of documentation this is, e.g. template, module, message, etc. This will be used as part of some messages the template might display, as well as categorisation.
The type is automatic based on the namespace, so it shouldn't generally need to be set.
page
The page where the documentation should be. This defaults to a subpage of the current page called "doc".
content
Allows documentation content to be specified directly, rather than on a separate sub-page. Useful for small page-specific sub-page templates, e.g. Explosion/table.
nodoc
If a template has no documentation and you don't know how to use it, add nodoc=1 to the documentation template on the documentation page. The documentation's background will become red to make it more noticeable, and the page will be added to Category:Templates with no documentation, if it's a template.
This is automatically set if the documentation page doesn't exist or is empty, so this generally isn't necessary to set.
baddoc
Similar to nodoc, this is used to mark templates that do have documentation, but it isn't very good. This can mean it doesn't have enough examples, doesn't explain all the functions properly, or doesn't explain the point of the template properly. The documentation's background will become yellow to make it more noticeable, and the page will be added to Category:Templates with bad documentation, if it's a template.