Plug your USB Audio/MIDI interface's USB cable into one of your PC's USB ports.
Install the appropriate software if necessary (many Audio/MIDI interfaces are plug and play, meaning that they'll begin the installation wizard upon plugging the device in).
Plug one end of your quarter-inch guitar cable into your guitar's input jack, and the other end of the cable into the USB Audio/MIDI interface's input jack.
Open your multi-track recording software program. Create a new track by selecting "Track" and then "Create New Track." Change the input on your new track to the USB Audio/MIDI interface. Press "record" and begin playing.
Adjust the volume on your USB Audio/MIDI interface by turning the volume knob up or down in accordance with the output level you're receiving from your playback.