To use the modified cassette player, first you will press play on the player and then switch on the L9110 circuit with the slide switch.
You can use the built-in speaker or a standard 1/8" audio jack cable to get audio out from your tape loop. If you use the headphone jack, you can send the audio signal through effects pedals like reverb and delay.
Turn the potentiometer to affect playback speed and direction. Press the button to get the warbling speed up effect.
The motor speed can also be used while recording to a tape loop. This can give you more or less recording time, as well as unique effects that you wouldn't be able to normally record.