Hi trbro
Try
here for some initial information. This question has also been asked before on these Forums so you could try doing a search.
I think you have to be professionally qualified (degree/diploma standard) and experienced, used to working with children and at least 30 years of age. You have to be able to play the piano to give the aural tests to Grade 8 level.
If you are interested why not contact ABRSM in London and ask them to send you an application form and more details?
Good luck if you do so.