Advice Please!: My SO has agreed to sit down and discuss his mother

Hello lovely people of JNMIL! My FMIL has finally crossed such a horrendous line that he finally sees her as the unnecessarily cruel being she has been to me for the past three years.

This being said, I am here for advice. We have agreed to sit down and discuss the game plan moving forward and how we can address her actions together, as a team. As I know many of you have been in similar situations, any techniques or ways to structure the conversation so that it will be most productive would be immensely appreciated. I already know to use “I” statements and to not use an accusatory tone as much as possible. I appreciate you all.

Advice Please!: My SO has agreed to sit down and discuss his mother