There's no app to download. Add Skybox to your home screen and it opens like one.

iPhone: open this page in Safari (if you came from an email, tap "Open in Safari" first).

Find Share. It is part of Safari, not a button on this page. If you see a square with an arrow, tap it. If not, tap the small icon at the left end of the address bar, or the three dots beside it, then Share. If no menu opens, press and hold the address bar.

Then scroll down, tap "Add to Home Screen", then Add. If you see "Open as Web App", leave it on.

Android: open this page in Chrome. Tap the three-dot menu, then "Add to Home screen" or "Install app", then Add.

Then open Skybox from the new icon and sign in once with your email and password.