I agree DEFINITELY use a nanny diary!
I always write in it every day and i think the younger the child the even more importnat it is.
As for a half day show nanny round your kitchen...
where you keep things
how washing machine / dryer / dishwasher etc work.
where you keep hoover / dustpan and brush etc
give her keys to house and show how alarm works etc!
show her where to find important phone numbers
where the stop cock and fuse box is
where you keep bedding etc
how the babies nursary is in order... ie what you keep where
how to collapse and lie back the pram... and unswivel the wheels and all the other fiddly bits
tell nanny the basic routine that you use... when / how long baby sleeps for etc
maybe have a cup of tea, and a chat and then leave her to it for a while
good luck!