In the first place, you need to ensure your device is properly supported (check out "Is the app supported on my device?").
Then, open the app and click on a category you may be interested in. Find a feature (or "tile") you want to enable, click on it and select "Enable tile".
Finally, follow "Where can I find the features I've enabled?"
You can also watch this video on YouTube
The app requires at least Android 7.0 to work.
It may not work on some unpopular brands, but it's worth to give a try. In case your smartphone or its brand is quite popular, 99% of the cases, the app works just fine.
If you're still having issues on a device of a popular brand, see "When I click on the tiles they don't respond."
I understand this is the most difficult part of the app to understand, trust me.
Scroll down the notification panel twice, until it's fully expanded. Then, you may find a pencil icon or an "Edit" button (try scrolling horizontally or opening menus). Once you've found any of these, click it.
Scroll down to the bottom: here you should see all the features you've enabled. Drag and drop them at the top of the screen, then close this menu.
All done, you're ready to go! Enjoy the app.
This is an issue which occurs on some brand's smartphones.
Go to dontkillmyapp.com and click on the name of the brand of your smartphone. Finally, follow the steps under "Solution for users".
In case the issue is still there, please send me an email.
You need to use a tool on your computer. Detailed instructions are available at this page (QS Enabler)
Since you're looking at that message, it looks like you haven't root permission on your phone.
Don't you know what root permission is? Read this page on Android Authority, it explains everything in detail. Anyway, in case you decide to go ahead and root your device, I'm not responsible for anything that may happen.
In case you're actually using a rooted device, disable any system which hides root or send me an email.
Detailed instructions about the procedure are available at this GitHub page