Plug the 3.5mm male end of the cable into the audio "Out" port on the side of your laptop. This port is typically used for headphones. You can find a 3.5mm male to 1/4-inch male cable at most electronic stores. Some of these cables have two 3.5mm connection cables; if you have one of these cables, you only need to connect one of the two cables into the laptop.
Connect the 1/4-inch male end of the cable into the line-in port on your guitar amp.
Power on the amplifier, then begin to play the audio from your laptop. With a cable connected to your headphone port, all audio is focused into your amp, and no audio will come from your laptop speakers.
Adjust the volume of the playback of the computer through the amp controls on the top of the amplifier.