If I am not mistaken, and this is an AHBA Herding Instinct Test you are talking about, it is very unstructured. Your dog does not have to know ANY commands. This is an INSTINCT TEST only. It is pass or fail. The experienced evaluator will help you, or will work the dog themselves for a period of time to determine if there is instinct present. That is it. Then you can say your dog has an HIC. A Herding Instinct Certificate.
The HCT is part TWO of the instinct test. Once you have a(n) HIC, part 2 of the test the dog must have a DOWN, and you must be able to call them off the sheep to demonstrate the minimum requirements of control. Once you pass this second part, your dog is HCT. Herding Capability Tested.
I believe you still have to have a PT, (PreTrial), before you can enter a herding test. I have not done any training for a PT, so I don't know what that involves, but I'm sure someone here does.
If what you are going to is an INSTINCT test, which is what it will be if your dog has never been on stock before, relax. Have a good time. It is some of the most fun I've ever had with my dogs. You don't have to perform anything, and neither does the dog. It either comes naturally, or it doesn't. And I will warn you. Your extremely well trained obedience dog may behave as if you are on another planet the first several times they are on sheep. I know mine all did. LOL
If it's the HCT, it's pretty informal too. If you have worked your dog with stock, and you have a down and a recall, you will do fine.