Decide whether you want to record your audio book with a digital recorder or with audio software. The advantage of software is that your MP3 file will be recorded directly to the computer. If you use a digital recorder, you will need to transfer your recorded files to your computer with a USB cord.
Break your book into chapters and consider recording each chapter as a separate MP3 file. It will make the recording process more manageable if you record in smaller chunks of time. It will also make it easier for your readers (listeners, in the case of MP3 audio) to manage their sessions with your audio book.
Find a quiet location to record your MP3 audio. Have your manuscript handy and printed in a large font to make it easier for you to read. Your audio file should be as clean and free of background noise as possible. The end result will be an audio file your readers can listen to. Anything but your voice reading the book will be a distraction.