The wiper (center leg) of the volume control goes to the point of the amplifier labeled, "input". One of the end legs goes to ground, and the other end leg goes to the signal you are sending to the amplifier. You will need to use an audio taper potentiometer, usually labeled something like 10kA (as opposed to 100kB). If the volume control seems to start real loud when it's only turned up a little bit and then doesn't change the volume much, you have the end connections reversed.
I'm typing this because it's easier than making a drawing, scanning it into the computer, re-sizing it, changing it to the right format, going advanced, browsing, uploading, and so on.