You would never
ask a potential client, or someone you will be doing work for, how to make a showreel. I could go into why, but that would take too long.
IMHO, Simon Ravn has one of the best demo reels out there. I actually modeled mine after his. Epic Sound - music, sound design and voice acting - listen
Click "Hear Demo CD" and then click any of the three on the right; "Orchestral" is probably the best one.
If you dont have the voice for that (few of us do), then hire someone. That's what I did. Obviously, you would need different text, which would have to be applicable to what is is you are doing.
Now, that may be overkill for a showreel which is just for a music lib, so maybe you want to skip the vo and give them just music. I wouldn't put entire tracks on the CD - do about 20 - 30 sec of each, and do not leave blank spaces inbetween each; crossfade the tracks, so the individual tracks become one long track. 5-6 tracks will be good enough; if they like what they hear, they will get back to you with a more specific request. Typically, you want to put your best track first, but if your best track is slow, mushy underscore, you wouldn't put that first. You want something fast, upbeat, that is going to grab the listener's attention, and pull them in. Give them a variety, but dont go overboard and give them 1 orch track, 1 rock track, 1 jazz track, etc. Variety is good, but there should be some semblance of focus on what it is that you do.
If you are putting a CD in the mail, dont just send the CD with some hand-written jibberish on the disc. I do that when I send CDs of new work to libs that I am already with; that's fine. But you want to make a good impression the first time. Make sure the CD and package look good. If you are sending them a web link, make sure the player can play the (elongated) track on any browser, and that it doesn't take too long too load.