Best place to start is by looking at her existing bike and try to mimic the contact points. Then ask her what she likes and doesn't like about it; what she is expecting from this bike. Adjust as needed from there.

Getting her input from the get-go will make her love the bike and appreciate your work even more. Trying to build a bike for one without their input often results in a lot of money rebuilding.

Curious, how did you settle on the Surly Troll?