Look through commercial copy and radio scripts for several that work for your type and vocal range. You can also write your own scripts or look through magazines at the ads and pull copy from these examples. You will also want to find some animated scripts with characters you could easily play.
Go through all your copy and pull out your favorite five to 10 items. Get a hand-held tape recorder and practice all these scripts in your various voices and accents. Listen back to them and continue to try to improve them. If one of the scripts isn't working, then forget about it and just work on the others.
Set up your voice recording equipment. This may be a microphone attached to a computer, a video camera with a mic or an actual sound studio type set up.
Record yourself acting out all the scripts. Do several takes of each one, and record an introduction where you state your name and agent (if you have one).
Import all the audio into your sound editing program (or video editing program that has the same capabilities). Choose the best take of each script. Edit them together leaving a two-second break between each script. Make sure to have your introduction at the beginning and end of the reel. Export the audio file.
Burn the track on a blank DVD and label with your information. You now have a voice acting demo reel that you can send out.