Hold the beer bottle by the neck, close to the cap. Grip the bottle tightly and make sure the condensation does not cause your hand to slip. If you are right-handed, hold the bottle in your left hand.
Holding the key in your right hand, wedge the key lengthwise, teeth side out, between the bottle cap and the lowest, largest section of your index finger. You may have to readjust your left hand grip higher or lower on the bottle neck to accommodate.
Using your finger as a fulcrum, push the key's head down with your right hand so the length of key engaged with the bottle cap puts pressure on the cap, ultimately popping it off the bottle. Apply slow steady pressure, rather than one quick effort.