4.  Express Your Feelings About the Situation.   You may want to use "I" Statements.

I get mad when you do these things because I feel you’re not considering my needs.   It makes me wonder if you respect me.

5.  Make Your Request for Changing the Situation.

I want you to ask me if it’s ok to use or eat something in the kitchen that's mine.  And I’d like us to agree to reimburse each other for food by replacing it in no later than two days after we've used it.

6.  State what will happen (the Consequences) if the person does or doesn’t cooperate with your request.

If things don’t change, then I won’t get you things from the grocery store when I’m going there for myself, and I won't cook meals for us to share anymore.   Depending on how things go, I may ask you to move out.