YASG Lightbox

Hello,
Can somone explain to me how to add a lightbox to a YASG gallery?

http://wordpress.org/extend/plugins/yet-another-simple-gallery/
says:

This gallery doesn’t use Lighbox, but it can be added easily by adding a link and a ‘lightbox’ class to the main image (Providing off-course having downloaded and activated the ‘lightbox’ plugin)

But I’m not sure what is meant by this. I have a few existing YASG’s on different pages but now want to add lightboxes to each one of them.

What is meant by “adding link” and “lightbox class”

Thanks a lot!