Download a free recording program. You don't need anything fancy; you'll simply be reading on one track. Do a search for free programs, download one and install it. See the Resources section for few suggestions.
Open the program and create a track. Label the track the title of the book. Familiarize yourself with the program you've chosen; most come with a user manual.
Plug the microphone adapter into the computer's speaker output. It should be compatible with a 1/4-inch cable. Plug the microphone's 1/4-inch cable into the adapter. Position the pop screen on the microphone. This will eliminate the popping of the "P" sound and reduce "S" sounds.
Adjust your program's audio output settings. This will be different for every computer. You'll know you're ready to record when you see the EQ line on the program move up and down when you talk into the microphone.
Press "Record" and read the manuscript into the microphone. (Alternatively, hire a voice-over professional to read it.) Stop frequently; every chapter is a good stopping point.
Export the file when done reading. Click "File," "Export Project" and save in MP3 format.