Open Logic and select "Empty Project" from the project selection screen that appears as Logic loads. Logic loads the blank project template and prompts you for a track type.
Select "Audio Track" and leave the other options as they are. Click "Create." Logic creates an audio track at the top of the Arrange Area.
Open Finder by clicking on its icon in the Dock and navigate to the file containing the vocals you want to distort and drag and drop it onto the track. Logic analyzes the file and displays it as an audio region on the track.
Click on the audio region, then click on the "Set Locators" icon in Logic's toolbar. This will cause Logic to loop the vocals, which will be useful for auditioning different distortion settings without having to constantly restart playback.
Click on the first empty box below "Sends" on the track's channel strip. The channel strip is the window to the left of the Arrange Area containing a fader, panning knob and other track-specific functions. Select "Bus 1" from the pop-up menu. Logic will now send audio from this track to an auxiliary bus whose channel strip will appear to the right of the track's channel strip. It will be labeled "Bus 1" at the bottom.
Click on the first slot beneath "Inserts" in the Bus 1 channel strip. Open the "Distortion" menu in the pop-up and select "Distortion." Logic loads the Distortion plugin on Bus 1 and displays its interface.
Click "Play." At its initial setting, the Distortion plugin will apply only slight distortion to the vocals.
Increase distortion by clicking and dragging the slider labeled "Drive" on the left side of the Distortion plugin's interface. At its highest setting, the vocals will become barely recognizable; at its lowest, no distortion will be applied.
Use the "Tone" knob on the Distortion plugin to make the vocals "brighter" or "darker." In digital audio production parlance, "bright" refers to sound with a lot of harmonic content; "dark" refers to sound with minimal harmonic content. Experiment with different "Drive" and "Tone" settings until you achieve a distortion effect you like.
Right-click on the audio region and select "Bounce in Place" when you are happy with the distortion effect. A pop-up screen appears. Leave the settings as they are and click "OK." Logic produces a copy of the vocals with the distortion effect applied, placing it on a new track and leaving the original file in its undistorted state. The distorted version will now appear in Logic's Audio Bin for use in any Logic project.