Extraction is heavily controlled by amount of water, temperature, grind size, and time.
To get an under extracted brew, you would use more water with a shorter brew time with a course grind at a lower temperature. This should give you something that's sour, weak, watery, with a thin mouthfeel.
Overextraction would be the opposite. Boiling water in a preheated brewing device, fine grind, less water, steeped for a longer time. This should taste dark, bitter, burnt, strong, with a soupy grimy mouthfeel.
You can make one cup of each at the extremes to taste the difference, and then use that information to tweak your regular brew one variable at a time. Look up the coffee compass to help understand what you're tasting and which direction to move.