I was wondering if somebody could tell me, from experience, what would be the best way to approach that and how not to spend 3 years figuring out what is what in the code and how it relates to other parts etc.
Install and develop with FireFox or Chrome if you haven't already. Chrome has this built in, but if using FireFox, install the FireBug extension. This will allow you to right-click any element and select "Inspect element." This will open a pane at the bottom of the window, the left half will be the HTML, the right, selectors affecting that element. It will say "style.css line 643" . . . and show the selectors.
Second is a really really good text editor. I still use HomeSite, but Adobe killed it when they bought Macromedia . . . haven't found anything that beats it yet.