It's been a while since I looked it up, but I'll get some step by step instructions put together for you in the near future, sorry for the delay in replying.
As mentioned by Crazyguy, there are a few pretty subtle ways to get donkey brain that you may be missing. However, there are a couple ways to reduce donkey brain as well, most notably, one of the spells in the spell book can reduce donkey brain a little with each cast. If you'd like specifics on those, let me know, otherwise I'll let you explore a bit more.