- Set up a development environment - see How to become a MediaWiki hacker page for instructions
- Choose a MediaWiki extension of your choice which is not converted to extension registration (that means a "No" in the "Converted?" column) from the list of extensions on wikimedia git.
- Check if there is already a task to convert this extension to extension registration, else create such a task as a subtask of T98668. Assign it to yourself and replace extension/skin in the task summary by the actual repository name.
- In your development environment, install the extension from git (note that initially it won't work) and migrate the code to use a extension.json file.
- Verify that the extension now does what it is supposed to do
- Upload the patch to gerrit and follow-up there with the feedback that you receive.
For reference, some previous patches to fix extensions are linked from https://phabricator.wikimedia.org/T98668