Can someone explain the whole "getting a mold" procedure, please?

You won't gag, the things just go over your teeth. They take two trays that kind of look like dentures with no teeth in. They fill them full of paste which just tastes kind of rubbery. Nothing horrible. They put them in your mouth like they would dentures and you bite down and stay still for a short while. It is painless. You drool a little. It tastes weird. No gagging, no pain, no fear. Easy.