GENDER APPOINTMENT

So this is my first pregnancy making it this far along, so I’ve never had a gender appointment before. Is there anything I should know? Is it possible that they won’t be able to tell what it is at the appointment? Also, I don’t want to find out at the appointment, I want my husband and I to do our own intimate reveal. Do I just let them know right away and they’ll put it in an envelope? What if I can tell my looking at the ultrasound what the gender is? Has that ever happened to anyone? Lol thanks in advance!