

Go to Settings → Mobile Data → Add eSIM, scan the QR code from your SpeedySim email, keep your home SIM as your default line, set cellular data to the eSIM and turn on data roaming for it. The whole install takes about two minutes over Wi-Fi. Every step, and the fixes for the common snags, below.
SpeedySim sends your QR code by email. If you open that email on the iPhone you're setting up, you can't scan the code with the camera. Two options:
Installing the eSIM and starting your plan are two different things. When you scan the QR code and add the cellular plan, the profile is on your phone. Your plan validity doesn't begin until you first use data in a covered country.
You can install a 15-day Japan eSIM three days before your flight. Your 15 days begin when you first use data in Japan. Installing early costs you nothing. One less job before you fly.

Get your eSIM sorted before you touch down