XP: maternity leave discussion/questions

I will be going on maternity leave in late June/early July depending on when my C/S is scheduled.  I have a wonderful relationship with my immediate supervisor as well as the one above her.  I work part-time (24 hours/3 days per week) at a medical school on a research study.  At the time of delivery, I will have been in this position 2 years.  

When I had DS I was not working (started this position when he was 17 months old), therefore, I am a bit clueless as to what all I need to ask, tell her, find out, etc. in terms of giving her information about my maternity leave prior to actually going on it.  Like I said, we get along very well and I know that she will work with me in terms of length of leave, etc. However, she doesn't have kids yet, so this is all a bit new to her as well. I told her that I would have some tentative ideas, etc. after winter break so that we will both be well-prepared.  In my position, she will be doing my job as well as hers, so I want to do my best to communicate as much as I can.

While I do have a bit of a list started, I can use all the info that you can provide for me.  What is important to ask/ get confirmed from employer?  Do I have to give a set time-frame/length of maternity leave prior to going out or can that be decided at a later date?