Watch videos of in hand classes then practice, practice, practice and practice some more. Not in looong sessions though, just stand him up for a few minutes at the end of every ride when he’s a little tired and standing still will be easy. Pay attention to where you stand when the judge is closely inspecting him. Watch those videos.
Gradually increase the time you ask for him to pose and stay still. Also practice letting him relax then resume posing, no horse is going to make like a statue through the entire class but they should be able to stay quiet while the judge inspects others…that takes practice.
See if you can’t get somebody to work with you on this, even a single session will reveal what you need to work on then you just need to practice a little every day working it in to your routine so its not a novelty at the show.
Thing with these classes, they are judged on the conformation of the horse. You got what you got. But by properly presenting and setting the horse up, keeping him posed and still while staying out of the judges way then staying quiet while others are judged you can certainly showcase what you do have.
Goes without saying (or should) spotless, well fitted bridle, well groomed horse including mane and tail, clean and neat clothing.
There are people who like to turnout in hand/halter horses, see if you don’t know one who can give you some hints. Its not hard, just detail oriented. Mind you, won’t move you up from 10th to 1st but might bump you up a few places against horses with similar conformation or give you an advantage in ties.
Those are very useful skills in horse handling and management anyway. Worth the time.