Undo the fixing screws on the rear of the amplifier case and determine two points on either side where it will be safe to drive the capped screws into the body.
Screw the screws into the case, making sure they don't contact any of the internal electronics. Hook the plastic caps of the screws through the holes at either end of the guitar strap.
Hang the amplifier over your shoulder like a record bag, so the controls on the front are facing up toward you. Determine a suitable mounting point for the speaker bracket. A C-shaped pivot bracket should be able to attach to the amplifier casing with a single screw. Ideally it should be facing away from you.
Connect the speaker lead to the output jack on the back of the amplifier. Connect the microphone to the input. You should now be able to speak through the bullhorn by pressing the "key" button on the side of the CB microphone.