How to setup Print.css

Hi there, I’m not well versed in CSS so I was wondering what exactly I put in the things like #header { display:none } so I can have my header and sidebar and such not get printed off. I tried things like #header { display:none } but the most it does is causes everything to get put underneath the header in the print preview while still showing the header. I use the Lightword theme if that helps at all.