The best practice is to create css files called whatever you like, then include them in the module/css folder, as you already have.
Then to add them to the module you need to add the function addInternalCSS('path') to the appropriate WebModule.php file for the module using the correct relevant path.
For example the line in the WebModule.php would look like this:
$this->addInternalCSS('/app/modules/[my module]/css/[my css].css');
I hope this helps.
Michael