Look in your notification panel for the confirmation that the download is complete. Tap the notification to begin installation.
Your phone will prompt you to allow unknown apps to be installed — either at a system-wide level, if you’re using an older version of Android, or for your browser only, if you’re using Android 8.0 (Oreo) or higher. Tap the “Settings” link in that box and then activate the toggle on the screen that comes up.
Hit the Back button, and you’ll see a prompt asking you to confirm that you want to install the app — either new or as an update, if you already have the app in place. Tap “Install.”
Once the installation completes, you’ll see a confirmation screen. You can open your app from that screen or by finding its icon in your app drawer.