If your iPhone is on iOS 17.4 or later, open your order page on that iPhone and tap the install button. iOS takes over and does the rest.
Otherwise use the QR code: Settings → Mobile Service (called Cellular on some versions) → Add eSIM → Use QR Code, then point the camera at the code on another screen.
If the iPhone you are installing on is the one showing the code, take a screenshot of it first. When the eSIM setup asks you to scan, choose that picture from your photos instead.
Do this on wifi, before you fly. An eSIM installs once, so do not delete it afterwards.