All of the tags and functions are defined in a JSON file located in the data/en folder.
Check the data/en folder to make sure no one else has already added it
Check the open Pull Requests to make sure someone else has not already submitted it.
Create a new JSON file, take a look at an existing function or tag and use the JSON format documentation for reference. Make sure there are links to official documentation (if there is no official documentation, explain this in the pull request comment)
Name it in all lowercase (eg awesomefunction.json or cfawesome.json)