On your iOS device, navigate to Settings > Apple ID > iCloud > iCloud Backup and turn on the “iCloud Backup” toggle. Your device will now automatically receive backups of your purchase history, device settings, photos and videos, app data, Home screen layout, ringtones, call history, HomeKit configuration, and Health app data.

Keep in mind that your device will automatically back up when its screen is locked, connected to a charger, and on a Wi-Fi network.