On iPhone it's two taps in Safari:

1. Tap the Share button at the bottom of the screen (top right on an iPad)
2. Scroll down and tap Add to Home Screen

SignRigs then opens from its own icon, full screen, like an app.

On Samsung Internet:

1. Tap the menu button (⋮ or ☰) at the bottom right
2. Tap Add page to, then Home screen

SignRigs then opens from its own icon. Or open this page in Chrome and tap this button there.