My experiment involved buying a can of whipped topping, and obtaining a large plastic freezer bag (a large Baggie). No no no, I did not stick my head in the bag. It wouldn’t fit anyway! The bag is way too small to put your head in, unless you’re a goddam mutant. What I did was stick the nozzle of the spray can deep into the bag, place the bag over my nose and mouth as best I could (with the rest of the can still sticking out), and shoot a blast of whipped cream into the bag while inhaling deeply. The cream fell to the bottom. I breathed the vapors. And I held my breath for a while (at least 15 seconds), to make sure the gases got trapped in my blood.
Personally I think you’d receive a far more effective happy gas fix from your dentist if you require it for this purpose though.