Most octopus sold in stores has already been cleaned, with all the contents of the head sac removed. You will still want to turn the head inside out where a slit has been made and rinse it of any debris such as sand and pull out anything that might have been left behind. Cut out the eyes, then flip the octopus over and you will see a small hole in the middle where all the tentacles meet. Pull out the hard beak by pushing it through from the other side. Make sure to remove any surrounding hard bits.