To be honest Julia, I found it difficult. I don't like confrontation.
With the one who fired us, I had had several talks about how she needed to improve in a couple of areas....and am ashamed to say that I lost my temper totally with her on 2 occasions. Once she had let the children scoot home (well over a mile) after swimming when it was minus 3 degrees with no gloves. Aged just 7 and 5. Another time when less cold she had ignored my instructions re scooting home (getting them to walk was a nightmare) because I had told her off after she had left expensive scooters outside the pool with no lock (the pool is v happy to have scooters inside, but she told me they weren't....) Another time she told me the boys hadn't eaten their veg which is unusual. After telling them off, it emerged that the cabbage was raw. I said that I'd have a problem with raw cabbage, mashed spud and fish fingers. Instead of saying, ok I'll do that next time, she launched into a load of self justification about how the French eat raw cabbage. Once when I had a bad day with my boss, she told me that I needed to tell him he had behaved unacceptably....yeah great in theory. It was never going to work with her - she had an answer for everything.
With the overweight one, I did have to sit her down and explain that I didn't expect her to eat 2 full meals everyday (my 12 salmon steaks were gone when I needed them....), and that I wanted her to walk (about a mile) to school once a day to pick up DS...I ended up paying for Weightwatchers. She was about 16 stone at the age of 22. To be fair to her, she was quite sweet and willing to try.
The one before last was a bit dippy but again, very keen to fit in. Talking to her did have a positive effect.
I think some you gel with and others don't....so approaching some will work well, and others will be a waste of time. The trick is to identify which sort you have so that you (i) fix it quickly or (ii) cut your losses early!
Good luck!