How to set up Notification Tracker for the first time

Setting up takes a few minutes and four taps' worth of permissions.

Steps

Install Notification Tracker & Alarms from Google Play and open it.
When the app asks, tap to give it notification access (see "How to give Notification Tracker notification access").
Allow the notifications permission when Android asks. This is what lets alarms and the app icon badge appear.
Choose Allow when the app offers to run in the background, so Android doesn't stop it.
Open the History tab. New notifications start appearing as they arrive.
Optional: open the Alarms tab and tap + to create your first alarm.

Notifications that arrived before you installed the app can't be recovered. History starts from the moment you grant access.
You can change every permission later in Settings > General Settings.