First, plug in your audio sources. You can use up to four and any device can be plugged in as long as it had a standard line level output.
Then plug your output into a speaker or other gain stage.
The individual input potentiometers affect the volume of the individual devices. The main output potentiometer affects the overall volume of all of the individual devices combined.
You could modify this build to use 1/4" jacks or mono jacks, depending on your needs. You could also add additional outputs, such as dedicated left and right channel outputs or aux outputs. Mute and solo buttons could also be added. There are a lot of options out there for you to create the perfect passive mixer for your DIY music needs.