To do a one-time Sass compile instead of a watch, you can do this from the command line:
sass --update scss:css
To have Sass import one file (usually a partial, with a _ starting the filename), you can do this inside a Sass file:
@import "_base.scss";
This way, Sass knows where you want the include to occur.
By default, Sass can’t import an entire directory. The Sass Globbing gem, however, can. You can install it from the command line:
gem install sass-globbing
And then watch with it:
sass -r sass-globbing --watch sass_dir:css_dir
Note that globbing will import files alphabetically, so be sure your CSS will cascade appropriately if this occurs.