What Buffy is wearing sounds fine - are you ok jumping in your dressage saddle? For WHP, you can get away with a saddle more suitable for jumping, although a straighter cut saddle is better for flat classes. You can quarter mark her - preferably do hunter style quarter marks, there is a post further down in this part of the forum which shows you how to make hunter style markings. She'll need to be plaited (mane only, pull her tail) and her feathers trimmed off.
You need to wear a tweed jacket, a navy blue riding hat (black will do if you have nothing else, but blue is more correct.), a cream shirt (put a couple of yellow Tesco value dusters in the washing machine with a white shirt if you don't have a cream one, and this will give you a lovely lemon yellow one), a gold/green/yellow tie (red sometimes works - experiment!) and either breeches or jods which are preferably lemon/canary yellow, although beige is absolutely fine too. You want either black leather riding boots if thats what you want to wear, or brown jod boots if you are under 18 or she is 14.2hh or under (sorry, not sure how old you are!). You'll also want brown gloves and a show cane, although you can get away with another stick for WHP if you need it!
