Whatever you do, make sure you are confident, stand up straight, head up, project your voice etc.
I'd probably start a convo with her based on her work, and then get onto a diff subject as soon as possible. Comment on something, tell a story, it doesn't matter what but just make sure you LEAD the convo. It's hard to explain but make sure it doens't sound forced or she'll feel uncomfortable, you have to try and be like the chatty guy you meet at the bus stop/bar,
At the end of the convo it's real simple:
'Hey you know what you seem like a really cool person, i'm going shopping at the weekend and i need a female eye to help me pick out some shirts, you should come. Put your number in my phone and we'll sort it out'
I can't stress enough though be confident, relaxed and don't act weird!
Eddy