Here is the general premise:
Take a remix made by another brony, and add your own "improvements" to it! It can be anything created by any other brony, be it Eurobeat brony, 8 bit brony, or even Iamnotacleverpony himself. (all found in the archive under their respective categories that no one knows exists!) The primary goal here is practice, so feel free to have fun with it.
For a full overview of all rules, including where to send these remixes (note: don't send them to me!), check out the google document found below!
Remix War Rules and Contact Information