From Clay Irving in rec.food.cooking
Date: Wed, 8 Oct 2003 00:48:32 +0000 (UTC)

Ceviche de Pulpo

1	kg	octopus	
8		tomatoes
1 or 2	onions
2 or 3	sprigs	coriander
		juice from 3 or 4 limes
1	small	green chili

Put a pan of water on to boil that is large enough for the octopus.  
Cut out the ink and eyes and throw away if this was not done in the 
market.  When the water comes to a rolling boil, grabbing the octopus 
by the top of the head, dip it legs first into the water 3 times.  The 
legs will curl up with this.  The third time drop the octopus in the 
water and boil until cooked.  It takes between 5 and 10 minutes and 
the tentacles will turn pink.  If you over cook it turns tough. 
Cool down.

Chop the tomatoes, onion, coriander and chili into a kind of salsa.  
Cut the octopus into bite size pieces and mix well. Add lime juice to 
taste.  Some people add a little tomato ketchup, too.  Serve well 
chilled with crackers on the side. Serves 4.

Basically, once you know how to cook the octopus, follow a guide like 
this:

http://www.felixcabosanlucas.com/news21.htm