Cytoid can be downloaded from the App Store, for iOS devices, or the Google Play store, for Android devices.
1. Go to cytoid.io and navigate to the "Levels" section.
Alternatively, you can go straight to it via https://cytoid.io/browse or simply use the search bar in the dropdown menu (you can see more info about searching by tapping the "?" button).
2. Find a level of a song you like and click on it. You can preview the song by pressing the triangle play icon in the lower right corner.
3. Press the "Download" button under the title of the song.
4. Wait until the level has finished downloading.
5. Tap the download notification. Cytoid should now open and import the level if everything is ok.
If you use Android and don't get the notification for some reason, you can go to your Downloads folder and tap the .cytoidlevel file there.