Connect a standard instrument cable to your guitar's output jack. If using an acoustic guitar with no output jack, you will need to attach an acoustic guitar transducer to your soundhole.
Connect a 1/4" to 1/8" adapter to the opposite end of your guitar cable.
Connect the guitar cable to your computer by inserting the 1/8" plug into your Line-In port. Line-In connections will not give you the high-level quality sound that a USB connection will give you, as Line-In connections transmit internal noise from the computer, but you will get far greater sound quality than you would by playing directly through your computer mic.
Set your computer's audio input source as "Line-In." If using Windows, click "Control Panel" and select "Speech," followed by "Speech Recognition" and "Audio Input." If using a Mac, click "System Preferences" and select "Sound," followed by "Input." Select "Line-In" from the available options.
Connect a standard instrument cable to your guitar's output jack. If using an acoustic guitar with no output jack, you will need to attach an acoustic guitar transducer to your soundhole.
Connect the opposite end of your audio cable into a 1/4" input on your audio interface. You can purchase an audio interface at any music supply store, choosing the right model based on your needs. Some interfaces include only a couple of ports for connecting instruments, while others contain entire mixing boards with multiple input sources.
Connect your interface to your computer by inserting the USB plug into a free USB port. If your PC has FireWire ports, you can also use a FireWire interface, as opposed to a USB interface.
Open a digital audio program. In order to take advantage of playback, recording and amp simulation features, you will need to use software designed for audio editing, such as Pro Tools, Logic, Reason, GarageBand or Cubase.