If you can find a groomer where you can sit and watch her do it, that would help a little. Mostly, it would give you a line to follow the next time she needs trimming and then I think you'd be more able to do it yourself. Right now, she's quite grown out and it would probably be very difficult for you as a first timer.
Here's another site:
http://www.groomers.com/breeds/yorkie.html