How do I get the gender in a sealed envelope?

Hi everyone. We at decided to hold out on finding out the baby's gender until birth. However, we cannot anymore! I've read about people getting the gender in a sealed envelope for their gender reveal parties. How do I go about getting that? I'm not sure I want to know it right now, but in case I do want to know I want to have it close by in an envelope. Should I give the u/s tech an envelope with a paper in it? Or do they provide it themselves? What should I ask her? Sorry I sound crazy!