Learn Korean words in Hangul
Hangul makes Korean vocabulary much easier to remember because pronunciation is built into the spelling. Practice reading the word and recalling the meaning together.
Turn words into phrases
Once a word is familiar, use it in short polite sentences. Vocabulary sticks better when it has a grammar pattern attached to it.
Questions
Should I memorize Korean vocabulary with romanization?
Use romanization only as a short bridge. Hangul practice is much better for long-term reading and pronunciation.
How many Korean words should beginners learn first?
The first 500 high-frequency words are enough to understand many basic phrases and beginner dialogues.