Be confident, knowing that there is nothing wrong with you, that you aren't deceiving anyone, and that you are attractive to your partner.
So tell them, but don't them ask for permission to be you. "I am", not "I think".
Chances are, you know a lot more about trans stuff then they do - and they'll realize that. So be the expert, at least about yourself. Let them know that you are glad to answer questions, but that essentially you are no different than anyone else. Be confident!
Also, you don't need to give long explanations. Be brief initially, unless they want to have a respectful and honest conversation in more depth.