reh!

If this is your folder structure:

Project 1
 |-- HTML
 |   |  index.html
 |
 |-- CSS
 |   |  style.css

Then in index.html you need to link it like this:

This is a relative path, so starting from index.html you have to go up one level (../) and go into the CSS folder ( CSS/) and select the file (here style.css).