It's as easy as creating her own account on iTunes. Transfer your old account to your new device, then create a new account in iTunes especially for her.
Note that you CANNOT transfer purchased items you paid for in your own account, to her account, unless the app has a "gifting" or "transfer" feature. Many apps DO NOT have this feature, in order for them to offer it to you at a cheap price. You'll have to buy a new copy of the apps, even if you no longer use them yourself anymore on your old account.