One thing that I guess made me and my roo stand out from the crowd in showmanship (I once had a judge training another judge and making comments to him) is how I set him down. I didn’t just plop him down, let go, and ask him to pose. I instead held on to him and kept him stable until he found his feet.
It would help you if he’s comfortable being tilted around while you hold him, even rolled briefly onto his back. He should also be comfortable with you touching his face and manipulating his legs while you hold him.
For posing, train him to pose using mealworms or another enticing treat and gradually make them stand prettily longer before getting it. Practice without a visible mealworm *before* show time. Clicker training can help a lot with this.
It’s super important that you’re friendly with the judge. Smile a lot and make tons of eye contact. You’re naturally going to want to focus on your chicken the whole time, but they’re ultimately judging you on your handling skills and those should be second nature to you by now- not something requiring all your attention. Don’t sweat it if your bird doesn’t behave- shows are stressful and this gives you a chance to show that you can hold on to and calm a flustered bird. Again, in showmanship, they’re mostly judging *you* and not so much your chicken.
You should know all about your selected bird. Breed, variety, class, other breeds in their class, the use/purpose of the breed, details from the breed’s standard, specific examples of ways your bird deviates from the standard, etc. as well as general info about chickens.
Don’t stress too much or expect a lot from your first go at it! Just have fun and soak in everything going on. Each show and judge does showmanship a bit differently so I recommend chatting with people who’ve been doing it there a while. Competition in my area at least was pretty friendly so they’d probably be happy to give you a better idea of what to expect.
This was pretty rambly, but I hope it helped!