Besides the standard specifications, tutorials you'll find everywhere I would recommend you to analyse the existing stuff, pages you especially enjoy, or those created by CSS gurus etc.
Just like with code or any other activity, it is great to try to mimic giants.
Also, two notes.
One. Don't know how about others but I've found myself rather unable to learn conceptually new things on deadlines or working on real projects. So, at least, don't feel bad if you won't deliver good stuff.
Two. There's a lot of "layers" in the UI: information architecture, usability, CSS, graphics, and perhaps some other I forgot to mention.
Just like with code or any other activity, it is great to try to mimic giants.
Also, two notes.
One. Don't know how about others but I've found myself rather unable to learn conceptually new things on deadlines or working on real projects. So, at least, don't feel bad if you won't deliver good stuff.
Two. There's a lot of "layers" in the UI: information architecture, usability, CSS, graphics, and perhaps some other I forgot to mention.