Talking to Japanese people, how to ask for permission to talk in casual form?

Question:

I'm hosting a Japanese student and I want to talk to her in plain/casual form rather than formally. How do I ask her that? Or do I just immediately start talking informally to her? I've emailed her a few times, the majority was in English but where I wrote Japanese, I used ます form.

Answer:

Ask this.

タメ語で話してもいいでしょうか？
tamego de hanashitemo iideshouka?

plain form = tamego