The best way to find long tail keywords is actually to start with the “head” keywords, and find out their related SEO inventory. If you’ve already started a startup or run an online business, then you probably have done some keyword research before. Put your keywords into the Google Search Console’s Keyword Planner and see how many searches they get. (Be sure to select “Exact Match” so you don’t include searches that include misspellings or synonyms.)
For example, let’s say I want to rank for the keyword phrase “weight loss". It's very hard to rank for such a phrase, so it's better to aim at longer phrases like "weight loss apps for android", etc.
At SUSO we usually focus on long-tail keywords and low-hanging fruits first, as they're the best place to begin rank spikes.