(→How to update existing /sandbox and /testcases subpages: On another wiki I didn't sync the sandbox of a template and the sandbox got deleted. Oh well, you can just recreate it.)
Tags: 2017 wikitext editor Reverted
To make a change to a template which already has /sandbox and /testcases subpages, the process is similar to the above.
# The /sandbox may not have the latest version of the template code in it, so before you make a change, you need to synchronize it with the main template. Copy the entire contents of the main template to the /sandbox subpage, and save the page. Your edit summary should be something like "synchronize with the main template", or simply "sync".
# Have fun editing the sandbox. When you have some code you want to test, save the page.
# If you have added any new features or changed any parameters, you should add new test cases to the /testcases subpage so that they can be tested. Also, bear in mind that the existing test cases may not be complete. It's okay to rewrite the test cases if they aren't useful for what you're trying to achieve, but also consider editors who may use the test cases after you.