How do you unlock the 2nd and 3rd part to a shout?

I should probably clarify, I'm talking about how you can put a total of three dragon souls in a perk. I'd like to know how to be able to spend 3 souls into a perk. some of the ones I have only let me spend one or two

bump

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You find the words. As far as I can tell its random so just keep searching those caves!

(mostly a bump)

Find the corresponding words and use dragon soul to unlock them.

In short, you learn new words of power by finding word walls in caves/dungeons, and when you learn a new word, you need to unlock that word's power by "spending" an absorbed dragon soul.

Assuming you're far enough along in the MQ, you can speak to one of the Greybeards (forget his name, but it starts with an "A" and he's the only one who actually has dialog). You can ask him if he's learned of the whereabouts for a new word, and he should tell you that you can find a word wall in . He'll mark it on your map too. (Note: I don't believe these are random; I'm fairly certain they're predetermined and hand-placed by Bethesda.) You'll then need to travel there, venture to the deepest part of that cave, and find the word wall. You should get a notification telling you the word was learned.

Now, you need to get a dragon soul (if you don't already have one). Find a dragon somewhere (they seem to spawn at higher elevations more often than lower, but that's an observation based on only a few hours of gameplay so it may not be true). Defeat it and you will absorb the soul. Now, open up your Shouts menu in the Spells page. Find the shout you want to unlock, and there should be an option called "Spend soul" or something like that. Do that and you should be good.