For this dish you are free to take the cheap "polipi non veraci" which do have only one "fila di ventose". They are less tender than the polipi veraci, but with this cooking method, it makes no difference.
You may cut them into pieces. put them into a pot and add slices of non treated lemon, also some green celery is good, you can carots too, anything what you imagine good with lemon flavor, obviously no patatoes, but I could try cassava next time.
Close the pot thoroughly to avoid that vapour comes out during the cooking process.
I close the the lid often with a paste of flower and water.

Before closing the pot bring it to boil, then let it simmer without ever opening the lid for about 90 minutes, done.
I never add salt before cooking. Sometimes they are salty enough from origin. You can always correct after cooking if salt is missing.
Already when taking away the lid you will feel the wonderful scent of the lemon. The dish will have a slightly bitter taste.
I served it on pan fried polenta slices, which was lovely.