Write a script. Yes, a demo can be scripted. However, don't just read right off of the script like a robot. It's there so you have a general idea of what you're going to talk about. Use it primarily as a guideline. Personality is what sells and the program directors will be listening for that.
Open your DAW or audio editor if you're doing voice only. Check the levels to make sure the microphone isn't too loud or soft. The environment should be quiet and a condenser microphone should be used with a foam shield to lesson plosive sounds that words that begin with P or B tend to make. They have better response and overall sound for on air-performance than dynamic microphones, which are used primarily for live settings and instrument miking.
Record the audio. Don't worry if you make a mistake. Just keep going. Record about 15 minutes, then review the recording. You can use small musical clips or sound effects for segues but make them brief.
Cut and paste the best sections into a new file in the audio editor. Edit out the mistakes that you made and other incidentals like loud breaths and background noise. Save the file and burn it onto a CD. Label the CD neatly with a marker and slip it into a CD sleeve, which should also be labeled.
Write an introductory letter and include your resume, preferably on personalized letterhead with a business card. Not only does this leave a good first impression, it will present yourself as a professional taking the audition seriously.