Practice before you do it. Make sure you have a good battery and it is charged. Don't lock your bees inside the hive when temps are in the upper nineties with high humidity.
Those are the lessons I learned from my first attempt a year ago.
In South Texas, it means you need to do the OAV in the morning, because you have to wait until it is really late to start the process.
In my case, I started when it was still hot and muggy. My battery didn't work. So after locking them inside their hive for 20 minutes and getting enough vapor to irritate them, I locked them in a second time and really pissed them off! The bees ended up inside my hood with me and told me just how much they hated me. They were grumpy for a week after that.