Tie your hair up in a pony tail 
Pull your hair through the doughnut and spread it over it 
Stretch a bobble over the hair and around the doughnut ring
Start to clip the strands around the base of the bun 
Make sure all the ends are clipped in and secure and spray it with hairspray
Ta daa! (I missed a bit which is annoying, grr)
This should work for any hair lengths as long as you can get your hair into a bobble and around the doughnut. It was easier when I had longer hair as the ends weren't so thick but it is still manageable! Sorry the quality of these gifs isn't great but the quality was reduced when I uploaded them.
