You do not have to go into too much specific detail with her regarding things....Just the important things such as what to feed the child, how much, how often, where the diapers/wipes/food/clothes, etc. are located, what time the nap is taken at, where and how to work the baby monitor, your general house rules, (I.e., T.V. time, visitors, answering the front door, etc.) as well as how to operate the washer/dryer/dishwasher, etc. if household duties are required.
Also, make sure to keep a pen and paper for your nanny so she can jot down any non-urgent questions or concerns for when you come home.
Leave her both you and your husband's cell phone numbers as well as your work numbers.
Ideally the pediatrician's name and phone no.# as well as the address would be great.
Also, a close neighbor to contact in case of an emergency.