how exactly does one use this module?

The module creates two sub-folders under your files folder:

files/customcssjs/css
files/customcssjs/js

Add .css and .js files to those folders. Voilà.

Thank you.
Alexis Wilke

To clarify: any files added to those directories are then included, on every page?

Yes. All of them. As if the module owned those files.

I intend to offer a patch to make that part better (i.e. use a path such as css/node/123/*.css and css/node/*.css). But that's not done yet. 8-}

That way you could very easily apply a style sheet to a specific page.

Thank you.
Alexis

Btw, you may also want to look at themekey. It does not allow you to keep the same theme and add a little bit of CSS/JS, but it could be a good solution, depending on what you're looking for...

Thank you for contributing to the module, I will update the documentation.