Practice parking and backing up. I went to a huge parking lot on a weekend when there wasn't much traffic for my first time.
The number one rule on parking and backing up is not to get in a hurry. Go a little, check your position, correct/do over, whatever, but don't get upset.
All the safety advice above is right on...I would add that you allow plenty of time and be sure to gradually speed up and slow down, especially careful around corners. It is sometimes recommended that you have someone drive and you ride for a short trip *alone, no horses* in your trailer so you get an idea of what it feels like to the horse.
Also, I *always* do a walkaround (like a pilot's pre-flight checklist) before hitting the road, to make sure all connections, lights, ropes/halters, and doors are safely hooked/shut/stowed where they are supposed to be. Learned that the hard way, when one of my tack doors came open on the traffic side during the ride down a narrow hilly country road with no safe place to stop. Yikes!
Lastly, I always plan my routes ahead of time and plan a contingency for if I miss a critical turn, especially in the country for trailheads - I want to know how to get back to where I need to be, and if I need to turn around, I want to know where there are good spots to do that. Saved my poor butt a couple of times that way!