Locate the source material. Get permission from the book or article's owner to record it as an audio book. If you're recording your own work, make sure that you've copyrighted the original version and the sound recording.
Practice the material before recording. Get acquainted with the pacing and vocabulary of the piece. Try different accents and be certain of how to pronounce names or difficult words.
Read slowly. If your speaking voice seems slow to you, it will seem normal to those listening. Enunciate and savor each word.
Modulate your voice. Avoid speaking in a monotone; change pitch and timbre to get your point across and keep the listener engaged. Don't speak in a rapidly changing "sing-song" voice, but let the material dictate your tone of voice.
Choose the best recording equipment you can afford. A simple mixing board will do. Check sites like Musicians Friend to find deals on four- or eight-channel mixers. Unless you expect to features lots of sound effects in your audio book, more channels aren't necessary. Invest in a good microphone, like a Shure or Telefunken.
Hire an engineer and voice talent for more complicated projects. If the book in question needs more than one narrator, or if you need to record music and sound effects, consider hiring voice talent and an engineer. If you don't have time to do so on your own, find an audio book producer to package your book for you.
Decide which product format you want to sell. CDs and digital downloads (MP3s) have replaced cassette tapes as the format of choice. Then figure out where to sell your audio book. You may want to use your own website, or sell it through a site like Audible.