You need to decide what you are prepared to do yourself and what you would prefer she did. I wash up, tidy everything away etc so that she can actually clean.
Main items for us are:
Bathroom, loo
Sweeping and mopping all downstairs floors
Cleaning public rooms (kitchen, sitting room) thoroughly
For me these are the basics, then you have the if/when you have time list:
Windows
My/kids bedrooms
Upstairs floors throroughly
Moving furniture
Books on book cases
I will leave a note if anything needs special attention.
Mine also will empty dishwasher, fold clothes out of the drier, put clothes into the drier. She doesnt tend to start a load, unless she made up the load (cleaning cloths and sheets for example). If I have something I don't want shrunk I make sure I deal with it myself.
Different cleaners take different amounts of time. Current cleaner gets paid to do the same tasks as the old one but does them much quicker (and better) so gets paid more/hour. We have found this to be an arrangement that works well for us.
I am a single mum with two jobs, so it is rare that I am there at the same time as her, and I generally try to make sure everyone is out when she is there so she doesnt have to clean around us. That said, if at all possible you need to be there for the first couple of sessions so you can answer questions and show here where everything is and what you want done.
We have a note book that we use to communicate with each other. It's only for us, and it makes it easier.
Occasionally I will ask her for extra hours so we can spring clean or whatever. She takes our ironing home with her, but as she comes twice a week it comes back to us pretty fast.