Short answer
Install the eSIM from inside the app, then finish setup in your iPhone settings.
Before you start
Make sure your iPhone supports eSIM.
Make sure the iPhone is unlocked.
Use stable Wi-Fi.
Update iOS if an update is pending.
Steps
Open the app.
Open your purchased plan.
Tap Install eSIM.
Follow the iPhone system prompts.
After the eSIM is added, open Settings → Cellular / Mobile Data / Mobile Service.
Make sure the new eSIM line is turned ON.
Important
The eSIM may show Activating before you arrive in the destination. Do not delete the eSIM while troubleshooting.
If installation does not start
Check whether the eSIM already appears in Settings → Cellular / Mobile Data. If it does, do not install it again; turn it on and select it for Mobile Data when needed.