Prepare a small pot of boiling water. Take the octopus. Wash it under running water until ink stops coming out. Look at its darling little tentacles.
Chop it in half just below and just above the eyes; this gets you a circle of tentacles, a patch of flesh containing eyes, beak and miscellaneous octopus viscera, and the body which consists of octopus-machinery inside an octopus-mantle.
If you aimed not far enough below the eyes, the beak is in the middle of the circle of tentacles, but it presses out easily. Ensure that you are holding the body in such a way that the octopus viscera do not get on the floor. Transfer it to over the sink. Mop octopus viscera off the floor.
Extract the octopus-machinery from the mantle - it's attached by one long linkage and three little things which I'd call spot-welds were the whole thing made of metal, so cut up the side of the mantle and then work round undoing the spot-welds. Do all of this under running water. Throw the octopus-eye section and the octopus machinery outside in the hope of befriending a new neighbourhood cat. Remind guests coming to visit that evening not to step on the octopus parts.
Cut out the tentacles (one at a time, from the middle outwards); cut each of them in half. Cut the mantle into a few pieces.
Put the tentacles and the mantle into the boiling water, stick a lid on it. Now start chopping onions.
Fry up two onions and some garlic in a casserole; drain the octopus, which will now have turned a nice orangey-purple colour, and put it in the casserole.
Add a tin of chopped tomatoes, put the lid on, and cook gently until the octopus stops tasting chewy; this seems to be about 90 minutes for my octopus.