My mum used to look after my DD for two days a week. I used to have a list written out, but it was more her routine, than do's and don'ts, but I saw her a lot through my maternity leave so I felt confident she knew the way I was trying to raise her, and what elements of her day were important to me, and which I was more flexible about. I think it's ALL in the way you say it. If you DC has particular toys or songs he likes, then you can tell MIL in a way that will make it seem like you are trying to make sure MIL has the most fun with her Grandson.
It's also fair enough to tell meal times and sleep times, if your baby is on a routine and that it's important to you - BUT, you have to make exceptions sometimes, especially if MIL wants to do a special day out. It's very different having a grandparent to childcare to a paid childminder or nanny - they are family and aren't being payed (presumably), and have their own relationship with your DC to develop as they see fit.
Do you have specific concerns over the way she might care for your DS? Have scanned post but can't see any examples of the kind of thing you would write. I think I remember putting something like "can you leave her to settle herself for 10 minutes at nap time before giving her a dummy, and please don't get her out of her cot and cuddle her, but stoking and patting is fine" . But then, I KNEW my mum wouldn't mind, and was glad of the refresher course and to be advised of changes in received wisdom on caring for babies since her day....she even bought her own copy of the Baby Whisperer - bless