Some tips from me:
If using scissors, use round ended ones so if you horse moves you don't cut her skin.
I would personally use scissors for her legs and face, I find you can get it a lot more natural looking.
Mix one part baby oil to three parts water in a spray bottle and spray her with it 3 days before the show. If plaiting her mane and tail, don't spray them. If not plaiting, spray generously as it will make her mane and tail really shiny. If plaiting her tail, on the day of the show plait her tail then spray lots of baby oil/water mixture through her tail and comb well. The result is a shiny, tangle free tail. Warning, if you spray on too much the tail goes all oily and weird so do be careful.
A damp sponge wiped over her coat on the day of the show will remove the top layer of dust and grease from your horse's coat and help shine.
Hope these are useful!