If you're using fresh octopus, you'll want to kill it humanely. (Makes it much easier to handle). The easiest way is to do so is to use a knife and stab it the space between its eyes.
Then use your fingers and flip the head inside out. Remove all of the entrails - be careful not to pop the ink sack. (If you do pop it, it's ok - simply wash it off).
Cut out the eyes out. Then remove the beak that's on the underside. Trim the tips off the tentacles as well.
Then give it a thorough wash. Let it drip off excess water.
Place the octopus into a mixing bowl and pour on some flour. Massage it all over - imagine your doing hand laundry. The flour will stick onto the impurities of the octopus. After 30 seconds of massaging, take it to the sink and wash off all the flour. Pour flour on again and repeat this once more.
Use a wooden pin roller or kitchen tenderizer and gently pound on the tentacles. This will help tenderize the meat.
Then trim off a few tentacles to use in your ramen. Cut the other tentacles and store in the freezer.