How do I add a JQL script function to JIRA?

[Peter Macdonald]

This probably sounds fairly basic, but the text information on the Custom JQL Functions page at https://<myJiraURL>/plugins/servlet/scriptrunner/builtin?section=script_functions# says:

If you have just added or removed a class for a script function, you can scan for them, otherwise they will be automatically loaded at startup.

But there is nothing that explains how I add or remove a script function class in order to then Scan it.