A good friend of mine basically told me that his ex needs a good guy in her life, etc. He is trying to hook me up with her. She's really cute, and I've been talking to her every now and then. Met her in person on 2 or 3 occasions. About 2 weeks ago when I was texting her, I asked her out on a date and she said "I would love to, but I have to let you know I'm seeing someone."

She just became single today, so I struck up a random conversation with her on Facebook about the job she got and starts tomorrow. She sort of segwayed it into her breaking up with her ex because all he wanted was sex and to party. I just played it cool and went along, didn't make myself look like an ass and was just myself. At that point she went into about how she's not gonna rush into anything, and I commented and said "there you go. smart girl. " The conversation ended after that and about 2 minutes later she signed off. The whole convo lasted about 45 minutes maybe and went pretty well, I think. It was very back and forth, and she agreed with just about everything I said. I have no idea how to go about sealing the deal and how to approach the next conversation, maybe make a move or something?

Send her a message, you can pretty much copy and paste this.

" I understand that you feel hurt and betrayed. 99% of guys can act like that. The kind of person you are is determined by how quickly you step back into life when someone you loved has shut you out. You are special and don't deserve to be treated liek a piece of meat. I believe that beauty is skin- deep and you are a great example of this. If you decide not to get back in an enjoyable relationship, he will continue to hurt you and that isn't fair. You can either sink or float, fall or fly. I am praying for you"

I used a variation of your message:
It got a pretty good response, I think.
The response wasn't perfect, as she bought into her ex's bull, but I just diverted the conversation away after. The convo went back and forth there for about another 10 minutes.
After that, my friend said she IM'ed him and was talking about how she still has feelings for her ex, which I understand. So I'm simply being there for her and earning her trust the old fashioned way. I know it's not gonna be easy, but she deserves much better than what she's getting and if possible, I wanna be the guy to show her that. My question is: How do I know when the time is right to start making my move?

Don't catch a crippling case of oneitis.

Play it casual with her, don't give up, but don't put your eggs in one basket friend.

I'm getting the feeling that she is doing exactly what I've seen before, and I'm sorry to say it, but it kinda seems like you're being a little too much nice guy and available.

The chances are low that she is going to change the circumstances whilst you are providing the support and comfort she needs, and her ex is providing the bad-boy attraction.

Open new girls and work on them, stay friends with her, but I think you should drop this thread about her ex and relationships and divert to having fun conversation.

If she isn't happy with him in one way or another and you stop making her feel comfortable about her relationship with her ex, but you are coming off as a fun guy with his own things going on I think you will have a better chance of attracting her.

Seems to me she goes for guys who do what they want and have their own thing going on.

exactly like Gman says, nice guy technique isn't really the best thing yuu can do..
i mean.. you don't want to behave like just a friend that is there for her when she needs to talk about her ex n stuff... you know, she has got female friends to talk about that...

a good thing is that yuu take her mind off that subject when yuu are in a conversation with her, find out her interests and make her comfortable talking to you. i mean, the last thing yuu want is her mind on that ex boy..

just talk about anything, but that subject.. make her feel good.. know what makes her feel good.. and i dont know, for example yuu ll find out that she likes to eat sushi, then make that an oppertunity to ask her out to have some sushi together when things go well ;p

just be a nice distraction for her mind, instead of confirming that her ex boy is a jerk and that she deserves better.. ;D but thats juss my 2 cents ;D