MediaWiki used to declare extensions in php files. Then MediaWiki 1.25 introduced a registration system based on a extension.json file. However, many extensions still haven't been migrated.

• 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.

A list of extensions pending to be migrated is available.

For reference, some previous patches to fix extensions are linked from https://phabricator.wikimedia.org/T98668