Yep, I’ve done it, but unfortunately my dog has lost interest. He’s got a high prey drive for vermin, but he needs to see the rat first to activate it before he’s interested in smelling it out, and he just doesn’t see the point when he doesn’t get to eat the rat at the end. My other two dachshunds might be able to do it at some point, but right now they’re too anxious in new places and with multiple people in the ring.
But it is fun when your dog has the drive.
There are often a lot of “fun matches” since it’s still fairly new and many places are getting started, so they have to host a certain number of fun trials before they can host a real one.
Instinct is very simple; the tubes are out in the open, and it’s just a matter of identifying the right one. When in doubt, you have a 33 1/3rd% chance of guessing it right, so if you’re getting to the end of the minute and your dog hasn’t indicated, go ahead and guess. 
I think my dog would probably do better if I did practice, but it is tough without rats. I would love to get a pet rat or two again, but my husband would kill me. There are starting to be more lessons and “rat ‘n’ go” practice sessions at local venues, so that’s one way of practicing without your own rats.
I do nosework, but my dog doesn’t make the connection between that and barn hunt yet–I’d again need more specific work with rats.
Like others said, just read up on the rules, but you should be fine with little to no prep for Instinct. It really is just a test to find out if the dog has the instinct naturally. But ask your local BHA club if they are doing any Rat ‘n’ Gos or lessons before the trial if you’d like a chance to make sure your dog can put two and two together. And have fun!