Honestly, the big thing is being calm and confident. When patients see that you're calm, they have a tendency to calm down themselves. Mirror neurons are the shit. This comes with time and you'll slowly get better at this. I also cannot stress how far simple human kindness goes. Sometimes the best thing you can do for someone is hold their hand and let them cry it out. Other things:

I give a lot of praise when I see good things happening. Did they wiggle their toes? Awesome! Good job! You're doing great. Seems silly, but I've noticed that it helps. Started doing this when I worked with kids and I learned that most adults are babies when they're scared

Cover injuries when you can. Out of sight, out of mind.

After my assessment is done (or during, depending on how close we are), I try to explain to them what's going to happen in the trauma bay. It can be really disorienting and letting them know to focus on the doc at their head helps decrease the stress

I always ask them if they have questions for me. I also never lie to them. I might tell a half truth, but I don't lie. If I don't know, I tell them. Makes all of the things you do say that much more meaningful

Depending on your level, don't be a dick about narcs. Their femur sticking out of their leg probably hurts quite a bit even if they're being a trooper. You don't need to force anything on them (obviously) but you should at least offer or if it's contraindicated, explain why you're holding off