The first week of any new App Store listing is the cheapest time you will ever have to rank for a keyword — the store has no signal on you yet, so a clean, tight metadata pass beats the spot you hold six months later when twenty other apps have piled on. Four passes, sixty minutes, you ship the listing by lunch.
Pass one: pull the five direct competitors whose screenshots you would put next to yours in a side-by-side. Read every word of their subtitle, short description, and the first three lines of each — those are the metadata they have actually A/B-tested into shape. Paste every term that describes the same job your app does into a single column. Duplicates and near-duplicates are fine; you will collapse them next. Roughly ten minutes, including the reading.
Pass two: open the App Store, type the single most concrete verb in your app (the thing the user is trying to do, not the category), and read the autocomplete. Every suggestion that comes back before your query is finished is a term real users type — and the order you see them in is roughly the order the store thinks they matter. Cross those against the column from pass one. The overlap is your core set; anything unique to autocomplete is your bonus set. Roughly fifteen minutes, mostly spent typing single words into the search box.
Pass three: read the last ninety days of one- and two-star reviews on each competitor. Phrases that show up three or more times across reviews are usually a problem to solve, not a feature to ship — but the nouns around them are the keywords the unhappy user searched for before they installed. They are the cheapest audience you will ever reach. If you do not want to read ninety days of complaints yourself, send the review stream to Bowroom’s review-themes dashboard — it clusters the recurring nouns across competitors and returns the long-tail you would have hand-extracted, in roughly fifteen minutes.
Pass four: subtract. Drop anything that is a brand name, anything that would force your app into a category your screenshots do not support, and anything that compounds (two-word terms where each word individually scores higher). What is left is twenty to thirty keywords you can ship in the 100-character keyword field — and a short list you will move into the subtitle and the first line of the description when you have ranking signal to spend. Roughly ten minutes; ship it.
Keyword research is one of six phases in the pre-launch to post-launch loop — the full 30-item sequence lives in the free Solo-Founder App Store Launch Checklist (mailed as a PDF, six phases, no fluff).