@perennialloner,
Your analysis is good, but it's still not exactly right.
We know that there are two types of Japanese adjectives:
1. i-adjective.
2. na-adjective.
For i-adjectives, if we want to use it as adverb. First remove the trailing "i" then add "ku".
An example of this would be the English sentence "They eat loudly."
Since "loud" is "urusai" and "eat" is "tabemasu". "Eat loudly" will
become "urusaku tabemasu".
For na-adjectives, if we want to use it as adverb. Just add "ni" behind the
na-adjective.
An example of this would be the English sentence "They speak quietly."
Since "quiet" is "shizuka" and "speak" is "hanashimasu". "Speak quietly" will become "shizukani hanashimasu".
As for natural adverb. We usually use it in isolation in a sentence.
An example of this would be the English sentence "Pretty hot."
Since "pretty" is "kanari" and "hot" is "atsui". "Pretty hot" will become "kanari atsui".
Good luck on your language journey!