How to Explain Your Ideas
One of the most important things a developer needs to do is explain what he/she wants to the person writing the music, or if they are instead aiming to find music, try to understand the mood you are trying to portray and express that in words. Specificity is a great thing, but try not to get too specific.
The easiest way to go about this is to follow a simple acronym (DEEP):
Describe how the character or player should feel.
Elaborate by explaining the feel of the environment the player is in with adjectives- dark? spooky?
Explain the environment in detail- only a few sentences though, no need to say too much! Plot is needed here, and concept art is good too!
Place the music in the plot relative to other musical cues and the rest of the story.
Also be sure to note the estimated duration, whether or not the track should be a loop, and any cues that need to happen at certain points (if it is a song).
If you have the urge to link examples such as pieces on youtube, note two things:
1. The composer, regardless of what you like and imagine the music as, will imagine the music his/her own way. Giving them your interpretation in a "completed" form is hindering them from using their full creativity. In addition, an example with a style that is unlike what you expect of them OR not familiar to the composer is likely of no use to the composer. EXPLAIN why you linked them to the piece!
2. Sometimes it is best to find pieces by the composer him/herself that are close to what you want and elaborate on where to go from there.
Often times, if you have a "this is JUST like what I want!" piece, it will lead to the composer being urged to copy the piece, which is not fun and can get awkward, aside from it defeating the purpose of even hiring a composer when you could just illegally rip the music you liked so much. Composers are there to add unique character and enhance your work, not make sound-alikes of your favorite videogame soundtracks at your pleasure.
I have had times where someone linked me to an old 8-bit song they loved and said it was like what they wanted me to write. I had to make it clear that my style was not compatible with that and if I were to write music for them, it would be orchestral in nature.
The Project Outline
Project outlines, or documents that outline the entire list of music you require (sometimes referred to as cue lists), are great. Follow the DEEP strategy outlined above for every piece and then send it to the composer(s). This is very important and will be the framework from which the composer will construct the score. The composer may wish to ask more questions aside from those in DEEP for further instructions, and some composers prefer more discussion than others.
Also be sure to outline: Optimal filetype (if you don't know, let them know what filetypes your game engine supports and ask for their opinion), estimated total duration, and any other information on the soundtrack (such as a general mood for the music or such). Also note if you want the composer to use a central melody to symbolize the character/game/story or not.
Here's an example of a good passage from a project outline (you might recognize this silly game concept from my other tutorial):
Making up a document like this and sending it to the composer is a major help in getting your ideas across cleanly and efficiently. This also makes those long back-and-forth e-mails and chats not as necessary and organizes all that might go on in one of those in one solid document. In addition, this won't get buried in your conversation and is much less likely to get lost somewhere as long as the composer is organized. You can also consider using a Google Doc or Skyedrive Document.
Another and also extremely useful way to present your plan is a spreadsheet version of the outline. This can compile things nicely and keeps you from getting too wordy with the limited space. I have had great success with this technique, and it's sometimes even easier than a project outline! The best approach is one similar to the outline below: