Connect a USB audio interface device (such as a Griffin iMic) to a USB port on your MacBook. Connect an audio cable with an 1/8-inch headphone plugs between the headphone jack of your analog music player and the "Mic" input of the USB audio interface device.
Drag down the Apple menu on your MacBook (in the upper-left corner of the screen) and select "System Preferences," then choose "Sound."
Click "Input," then select the USB audio interface device from the list of options. Click "Output," then select the internal speakers of your MacBook. Click on the red button in the top-left corner of the "Sound" window to close it.
Double-click the Garageband icon in your "Programs" folder to launch the software.
Select "Create New Music Project" and enter a name for your recording in the field provided, then choose "Create."
Close the "Keyboard" window if it appears by clicking the red button on the pop-up window. Drag down the "Garageband" menu and choose "Preferences," then click on "Audio/MIDI." Choose "Built-in Output" for the "Audio Output," and select the USB audio interface device for the "Audio Input."
Click "Advanced" at the top of the "Preferences" window, then adjust the "Audio Resolution" to your preferred level of audio quality. Click the red "Close" button at the top-left of the "Preferences" window to close it.
Drag down the "Control" menu and deselect the "Metronome" setting. Drag down the "Track" menu, then choose "New Basic Track."
Start your analog audio player with the volume set at about 80 percent of maximum. Check the animated color input level meter in the "Basic Track" information and make sure it reads no greater than 80 percent of the meter's maximum signal strength, adjusting it downwards with the recording level slider if needed.
Click the round, red "Record" button on the "Basic Track" to prepare to record, then return your analog music player to the point where you would like to begin recording. Click the "Record" button again to begin recording, then start the analog music player.
Click the "Play" button to stop recording. Drag down the "File" menu, then select "Save" to save your recording session to your MacBook's hard drive. Drag down the "File" menu and select "Export to iTunes" to convert the file to an MP3 that can be played on a portable digital music device like the iPod.