My little one figured out how to do her own bun for "bun day" practices at the age of 8 very simply, but it stays put! She does a pony tail, sprays the hair, combs through and divides into pony into 3 and just does a braid. Sprays again, wraps braid around base of pony tail, secures with an elastic, then a few bobby pins, sprays again, then plops a scrunchie over the top. It always looks great, and she can do one herself in under 10 minutes start to finish. If it seems overwhelming, explain to your mum and she may be able to visualize. DD has thick hair, so for meets I do the same thing, but make 2 braids and wrap one around first, secure, then wrap the second and secure. This proably isn't necessary- but I would hate for her to have an issue during a competition.
Even if you don't need a bun- I think they look wonderful on gymnasts. Good luck.