this helped me: https://www.w3schools.com/html/html_responsive.asp . sections "Setting the Viewport" and "Media Queries" i feel like are the most important. media queries basically let you change the styles you enter if the screen width gets too narrow
I'd say don't get intimidated by the scope of the project. Maybe build some new pages to nail down your methods and styles, then convert old pages when you're feeling zen.
TYSMMM!!!! <3