Set up your hardware and software. Connect your microphone to your audio interface, your audio interface to your computer and your headphones to your interface.
Start a new session in Pro Tools and create new audio tracks. Depending on your version of Pro Tools, the "New Track" option will either be in the "File" or "Track" menu tool bar. Check the track settings to ensure you are creating mono audio tracks, and create as many tracks as there are a capella vocal parts.
Test your microphone signal and set the levels. Speak or sing into the microphone and use the gain and level controls to ensure the vocals are not peaking and are not too quiet. Make sure the level in your headphones is comfortable and that you can hear your voice without it being too loud or quiet.
Add a click track or music guide track if you need assistance keeping in time and in tune. To create a click track, use the "Create Click Track" option in your "Track" menu. Use the "Import" option to add music guide tracks to your Pro Tools session.
Record your first vocal part onto a record-enabled track by pressing record and singing the part. Record as many takes as are needed for you to be happy with the recorded vocals.
Record the remaining vocal parts on each of the tracks you created.
Mix the vocals. Mute the click track and guide track while you equalize, compress and add reverb and other effects to each vocal part. Set the levels for each vocal track using the track faders and pan them to create a stereo mix.
Export the a capella song as an MP3 or WAV file using the "Export" option in the "File" menu.