Think of performance classes like real life scenes. Just like halter classes you need to start with a model to show. Although model condition does not count as much as in halter, it is still very important. If it comes down to tie between two entries the condition of the model itself will be looked at very closely.
You then need to decide either what the horse will be doing and use appropriate tack for that job, or what style tack you want to use and have the horse doing something appropriate for that tack. Keep in mind the horse breed must be appropriate for the task. A beautiful Arabian costume will not be right for a Quarter Horse.
Also, make sure all the tack is correct. Various competitions have specific tack rules. In real life you could be disqualified for using the wrong tack. Does the tack properly fit your horse? Again, keep in mind real life. It would be unsafe and/or uncomfortable on the horse if the tack did not fit right, therefore, you would be marked down for this in a model show.
Next is the fun part. Props. You don't have to have them but they are fun to work with and they add to the reality of your entry.
Are you in a show arena? Perhaps you are out on a trail or in a parade. Use the correct footing for your entry. What about a fence? There are many types depending where you are. Cones, markers, timers, barrels, trees, rocks, grass, animals, people. There are so many things you can add. Just be courteous to other showers and don't use more space than necessary for your entry. You can check with the show "host" for how large your entry can be, but no larger than 18" X 30" is a good to keep in mind.
What about a rider? Make sure the riders attire is appropriate too. Some events require specific rider attire as well. Be sure the rider actually looks like it is riding. Are the hands in the correct position? How about the legs and feet? All this counts.
Except in dioramas, you are not required to have any more than your horse and it's tack. All the additional props can add to the reality but if they are not correct for the scene, or if they are out of scale, they will count against you. If in doubt, leave it out.
The thing to remember is to do your homework. Research, research, and more research. Use real horse sites or books to help you. Visit local horse shows and get ideas. Talk to people who show horses or participate in events.
Most of all, relax and have fun!