Describe your children, their ages, their personalities and the things they like. Make them sounds like children a nanny would enjoy working with. Describe any past experienced you and your family have with nannies, and how you treat them. Describe what you expect, in detail, from the nanny. Describe the hours and the pay. Describe the time frame you have planned out for interviewing and how quickly you plan to move through the process. Tell when you want to have someone start ideally. Tell how to contact you.
I think this is a pretty good list except for the bolded. That sounds too close to gossiping about past nannies, which can get awkward and inappropriate if you had a bad nanny. Besides, who is going to say in an ad they didn't treat a nanny well?