Build a page layout with left sidebar and right content

Hi,

I want to build a page layout that contains the left sidebar and add the page content to the right. I tried to remove this:

<?php get_sidebar('right'); ?>

In the template-page-three-cols-slm.php file. It removed the right sidebar, but now I’m stuck with a left sidebar and a tiny content, and a blank space on the right. How can I make it so the left sidebar is say, 20%, and the content 80% width? I tried to edit the css but no success.

Can you please advice?

Thank you so much!