Communication: How to do it?

So I'm an ex-forever alone guy (24, 2 sexual partners) that is starting to get into a relationship with a pretty awesome girl. We've had sex a few times, but the first several consisted of me failing miserably (unable to get it up, premature ejaculation, etc.) More recently, I've improved from the completely disastrous showings and have actually been able to have sex for a few minutes. Hooray!

Now that I'm getting a bit more confident, I'd like to start talking to the lady about what turns her on and what we can do to improve in the bedroom. In particular, I'm interested to know:

1. She doesn't seem to like me going down on her. Do I suck at it, or is it something else?
2. Is there anything she really likes that can make her orgasm?

More generally though, after reading this subreddit for a while I've learned that communication is important to a good sex life. Outside of sex, I feel like we have a very open and honest relationship, so it shouldn't be too much of a stretch to talk about what goes on in the bedroom. The problem is, I'm just not sure how to do it. Is it better to bring it up before or after sex? Or in a completely non-sexual context, like when we're watching TV? Is there a certain way to broach the subject? I don't want to make her feel insecure or uncomfortable, so is there certain benign, non-offensive language I should use?

TL;DR - Need a step-by-step walkthrough on how to talk about sex with new GF.