How do you let you mum-in-law know that you will be calling the shots on the upbringing of your child (without sounding rude)?

If listening doesn't tax you much then listen to whatever she says, and you do what you want to. And if you don't like to listen then make some excuse and go away whenever she brings up the topic of upbringing or what one should do or not. Also you can make your husband part of this. Tell your husband that you do not like anyone preaching you be it is mother or yours and let hime handle his mother.