Unzip the download — you'll get index.html, manifest.json, sw.js and 3 icon PNGs.
Drop that folder on a free static host: Cloudflare Pages (Create project → Direct upload), Netlify Drop (drag the folder onto app.netlify.com/drop), or GitHub Pages.
Open the resulting URL on your iPhone in Safari → Share → Add to Home Screen. (Android Chrome: ⋮ → Install app.)
Must be served over http/https (not opened as a file) for the install + offline features to work.