Connect the microphone, guitar, keyboard or other device that you are going to output through the Pennsylvania amp to one of the inputs on the mixer. To connect a guitar, for instance, attach one of the 1/4-inch instrument cables from the 1/4-inch output on the guitar to one of the 1/4-inch inputs on the stereo mixer.
Attach the second 1/4-inch instrument cable from the main output of the stereo mixer to the 1/4-inch input on the Pennsylvania amplifier. This will vary depending on the make and model of mixer you are using, but look for a 1/4-inch output connection on the mixer that is labeled "Main Out," "TRS Out," or "Balanced Out." Make sure to use this and not any of the other 1/4-inch output connections as this will ensure that any level adjustments and special effects you add on the mixer will output through the amplifier.
Test the connection by sending signal to the stereo mixer and listening for the same signal on the Pennsylvania amp. To recall the earlier example, play the guitar you have connected and see if it outputs through the Pennsylvania amp. If so, the "fader" for that channel can be pushed up and down to increase or decrease volume, and special effects can be added to your heart's content.