Sage is an excellent starter theme for WordPress. It comes with Bootstrap connected out-of-the-box, but it does not have a “standard” Bootstrap nav markup. The old Roots theme (Sage’s parent) used to have a NavWalker() class that extended the WordPress Walker_Nav_Menu() class to give cleaner menu markup and Bootstrap friendly classes.
To add the NavWalker() class, from your theme root, run:
NB: see update below for a potentially better option.
Then replace the code in your templates/header.php with this (or similar Bootstrap-style markup):
If you are using the navbar-fixed-top class on the navbar element, you’ll need to add the following style rules:
Update September 2016
The original article didn’t provide the license for the NavWalker. I’ve now included the license in the file docblock.