Huge n00bish question, particularly since I've been here for a while now, but I never thought of customizing my profile 'till now.

I've got the code for a new profile, where do I put the code so it'll, you know, work?

Hover over the Personal tab > Site Preferences > Email C:

OK, that's all I needed to know. Thanks :)

you can also create a blank section on your profile and stick all the CSS code in there

This has always confused me, too. I know it's in site preferences but I never really think to go to the Email tab to update my profile and I end up clicking other tabs first. :P Hopefully they'll rename the tab or sort this out at some point.