The list is legally embedded within their Longman Dictionary of Contemporary English (LDOCE) . In the printed dictionary, these words are marked with a frequency symbol (a circle with a number: 3, 4, or 5). In the premium app, you can filter search results by frequency.
You could study the entire dictionary, but that would take years. You could rely on school textbooks, but they often miss the high-frequency words used in daily TV shows, news articles, and casual conversations. Longman 5000 Words Pdf
If you download a community-made PDF, it is an incredible roadmap. You can test yourself: If you don't recognize 20% of Tier 4 words, you know exactly where your weak spot is. The list is legally embedded within their Longman