To create a patch, first paste the original file (without changes) to the left textbox.
Then paste the file with the changes applied in the middle textbox.
Now the patch is automatically created in the right box. You can copy and paste it to clipboard or share the page via an URL link.
To apply a patch, first make sure that you only have the patch in the right box (via paste or via link).
Now past your current file in the left textbox.
Now the middle textbox is filled with the patched file contents that includes the changes.
Beware: If the patch is based on a different original file than you have, there can be conflicts that are not correctly resolved.
Please check the end-result for correctness and unintended changes.