Some tips on preparing and cooking octopus:
Firstly, you can use the old fashioned / traditional method of flinging it against a rock to tenderise it, or beating it lightly with a meat mallet (remove insides first!), or (if you've bought it fresh, rather than pre-frozen) you can just clean it up, preferably remove the insides first, then freeze for at least 48 hours up to a week (obviously it will keep in the freezer for much longer than this, until you wish to use it) to tenderise it.
When you buy your octopus, you can ask your fishmonger to remove the innards, eyes and beak for you, if you wish (which is what I do - even the supermarket fishmonger will do this for you - plus then you're not paying for weight you're going to throw in the bin).
If you have your octopus intact, there is a great video here, showing you how to do this (and also how to section the legs, if you're not sure).
Once that's done, you should be left with the head and intact tentacles, like so.
Pull off any loose skin or membrane from inside and outside the head, you don't want anything attached inside (you might need a sharp knife to help you with this) and wash the head and tentacles thoroughly, making sure that no hard parts are left on any of the suckers.
There are many opinions on cooking it, braising it in its own juices, cooking it sous-vide, 'shocking it' by dipping it into boiling water a few times, to make the tentacles curl first (they curled anyway, I found!)... I just wanted something simple, and effective.
So I brought a large pot of lightly salted water to the boil, turned down the heat, added the octopus section, and simmered it gently until the meat was tender when pierced with a fork, in the thickest part of the tentacles.
Some places recommend 45 minutes to an hour, I found it took over an hour and a half before the texture changed, and I gave it nearer two hours to be satisfied it wasn't going to get any more tender (checking it frequently, as I didn't want it to become octo-mush).
If you want to then remove the skin from the tentacles, which becomes quite gelatinous, it's best to do it while it's still hot (but cool enough to handle). If you're barbecuing it, you could leave this step, if you prefer.
They will look similar to this when they come out of the pan.
All I did, was to hold one end, then draw my fingers down to the bottom of the tentacle, not going over the tentacles, and it just came away really easily.
If you wish, you can also remove the suckers. I didn't, as I was intending to pan-fry them until they had a little colour and crispiness here and there, but for presentation in salads etc., or for dishes where you're not crisping or browning them at all, you may want to, as they're a little firm. They are easy to remove - you can just push them off with your fingers (a bit like podding peas!) or use a knife to scrape them off.
There are lots of recipes out there, from salads and pickles, to barbecues and sauteées...
You could try them in my Octopus Sautéed with Garlic, Lemon, Capers and Parsley which is an excellent way to use the head, cut into strips, and very quick to cook.
Or alternatively, really push the boat out, and use the tentacles in this dish, which (now you've got the time-consuming part over with), is surprisingly easy to make, and looks (and tastes) rather impressive! My Octopus, Chorizo and Red Peppers with Cauliflower Two Ways.