Acquire 1 ripe and juicy banana.
Place your hand on the posterior end of the banana close to the stem, then grip the banana using a US continental forehand grip as shown in the picture.
Your banana gun is now ready to shoot, keep in mind you should never aim it at someone's face as this could result in blindness if said person gets shot in the eyes. Use at own risk!
Add details as desired but be careful not to destroy the internal structures on the inside of the weapon as this could result in a malfunction!