Gender reveal help!

Chelsea • LRK 5.30.16💖 MGK 7.5.19💙

So with our daughter DH & I were too impatient to do a gender reveal. But this time, I want my sister to arrange a gender reveal for us and have our daughter open a box or pop a balloon, etc. so it’s like she is telling us. My question is: how do you arrange that with your doctor or ultrasound technician? Do you just have them write it down in an envelope or have them call my sister? I’m confused