This is already an advanced beginner task (we also offer beginner tasks that are way easier and take less time, but this task is very useful if you plan to continue with coding related tasks.)
- Set up a Vagrant instance (your development environment) - see this wiki page for instructions
- In your development environment, install a MediaWiki skin of your choice and which also does not yet have a screenshot on the extension's home page on mediawiki.org
- Make a screenshot of the skin (if possible, please only display the content within your browser window, and do not include elements of your browser software itself or even your operating system) - read more information about good screenshots here
- Upload your screenshot of the extension to Wikimedia Commons (use "MediaWiki screenshot" when a license is asked)
- Insert the image into the home page of the skin (use the |image = parameter of the Skin template)