The stirrups are fine to use.
You do need to wear the hunt coat and shirt unless this show specifically says they are not necessary. Is there anyone else you can borrow clothes from? If you are planning to continue showing you might want to get some of your own – check your local tack shops, a lot of times they will have used show clothing on consignment and you can get a pretty good deal.
You should always be on the correct lead. A simple change is fine in a schooling show and this is what you should do. Make sure you do them at the right time, not too late, with as few trot steps as possible. Obviously a horse that does the flying lead changes does have an advantage over one that doesn’t… but they won’t win just because of that.
I would practice walk to canter transitions this week. For a hunter under saddle class you want a long and low frame. Long, sweeping strides that cover the ground – but not fast. You also want him to be very consistent – not slowing down then speeding up or shortening then lengthening. Part of this is his natural movement, which you can’t help – but your job is to ride him so that he moves the best he can move.
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