application.css.scss or application.css are kinda the same. Just rename your application.css to application.css.scss.
As for adding that line, it’ll need to be right up the top, in a comment. Like this:
/*
* This is a manifest file that'll be compiled into application.css, which will include all the files
* listed below.
*
* Any CSS and SCSS file within this directory, lib/assets/stylesheets, vendor/assets/stylesheets,
* or vendor/assets/stylesheets of plugins, if any, can be referenced here using a relative path.
*
* You're free to add application-wide styles to this file and they'll appear at the bottom of the
* compiled file so the styles you add here take precedence over styles defined in any styles
* defined in the other CSS/SCSS files in this directory. It is generally better to create a new
* file per style scope.
*
*= require_self
*= require colorbox-rails
*/
@import "bootstrap";
@import "welcome";
@import "sessions";
@import "users";