Back in the 1980's we showed a couple of our dexters regularly. We had no previous cattle showing experience, only welsh ponies. We went to Herts County and watched the cattle being shown (no dexter classes as far as I recall). We saw the handlers wore white coats, and noted what was required of the cattle. I entered our heifer in the appropriate class at East of England. She was well handled, looked well, her feet were in good order.
So having bathed her the night before, and having purchased a white coat, we just turned up. All the dexter people were so friendly it was a joy. Someone came across and trimmed the hair at the top of her tail, another produced some sand paper and oil and showed us how to do her horns. We came home with a 3rd rosette as far as I recall.
My advice is just go for it. I am sure things could not have changed that much. As you say you have experience of showing in the 70's so you are better armed than we were back then!! Good luck.
Jean