git workflow for D

[bitwise]

I've finally started learning git, due to our team expanding 
beyond one person - awesome, right? Anyways, I've got things more 
or less figured out, which is nice, because being clueless about 
git is a big blocker for me trying to do any real work on 
dmd/phobos/druntime. As far as working on a single master branch 
works, I can commit, rebase, merge, squash, push, reset, etc, 
like the best of em. What I'm confused about is how all this 
stuff will interact when working on a forked repo and trying to 
maintain pull requests while everyone else's commits flood in.

How does one keep their fork up to date? For example, if I fork 
dmd, and wait a month, do I just fetch using dmd's master as a 
remote, and then rebase? Will that actually work, or is that 
impossible across separate forks/branches? What if I have 
committed and pushed to my remote fork and still want to merge in 
the latest changes from dlang's master branch?

And how does a pull request actually work? Is it a request to 
merge my entire branch, or just some specific files? and do I 
need a separate branch for each pull request, or is the pull 
request itself somehow isolated from my changes?

Anyways, I'd just be rambling if I kept asking questions. If 
anyone can offer any kind of advice, or an article that explains 
these things concisely and effectively, that would be helpful.

     Thanks