Consider working hours... Start time, finish time. Some people don't mind working long hours, others prefer shorter hours. Decide what you need and what you can afford.
You say you will be home at times and not at other times... some nannies won't mind that, others may want to feel they are in sole care. Reality is that these days nannies need to fit in with their employer, however your nanny will most likely want fixed hours - so decide on what hours would work.
I can often have a boss around at home, they may be working, resting, gardening, tidying up, studying etc. It works well most of the time.
There are many message threads on here about the practical side of things like Holiday entitlement, payroll, that kind of thing. So have a look through, or ask a specific question.
Consider what your children might need in 6 months, a year, two years. A nanny who is great with babies may not be as good with toddlers, though many nannies have experience caring for a vareity of aged children.
Outings can increase the cost of having a nanny quite a bit but also may mean your child gets to do things you wouldn't do with them. Your nanny can help your child learn about new things - I've asked children what they would like to do, got a one word answer such as Castle, Helicopter, and taken that as the thing to do - so visiting a Castle, going to see a helicopter up close, sitting inside it. Haven't yet been flying... that takes it a bit far perhaps.
Consider school holidays. Nanny will need to be able to care for all children at the same time and find things that with luck all will be interested in. Babies/toddlers luckly tend to like anything, so go with the flow most of the time.