Writing can be troubling at times, especially if the writer has so many ideas all at once. One of the most annoying things to happen for a writer is having so much in mind but having trouble forming them into words; the same goes for contest holders who have a difficult time in writing a good design brief for a contest.
Firstly, you need to think about what exactly you are looking for. Would you like something related to a specific theme like warm colors in order to give out a bright atmosphere? Perhaps you prefer a word cloud instead, composed of various fonts and shades as they are grouped together to form the final image?
Once you have thought about what you want, you can proceed to break it down to the essential details: Are there certain font styles you want for it or will anything do? Would you rather leave the color choices to the designers or would you prefer something uniform for all?
In case you suddenly decide to just leave it to the designers, you must be prepared to accept the fact that the designers will rely on their own skills and make do with what they can, and that in itself may leave you disappointed if you do not get what you are looking for.
You may choose to include a sample sketch or image, perhaps as a reference on how you want the design to look; this will at least narrow down the entries into something close to what you want.
To sum it up, think about what you want the final output to look, then list everything else from there. We wish you the best of luck!