Just insert your audio cable (red and white) into channel 1 or 2 (phono/line input) at the back of the controller. After that you need to set the channel to phono. This is done at the front of the controller. The selector will standard be set to ‘pc’. Just switch it to ‘phono’ and your good to go.
Channels 3 and 4 can’t be used for turntables.