Android
In the 'Make music' section you'll find a microphone icon in the options on the left-hand side. When you enter the microphone/recording mode for the first time, there will be a pop-up advising that a recording/latency test needs to be performed. You can then arm one of the tracks/channels for the recording and start the actual recording process by tapping the microphone button on the desired target track. A brief pre-count will take place, before the playback of the backing track you have put together and the actual recording starts.
iOS
In the 'Make music' section you'll find a microphone icon in the options on the left-hand side. You can then arm one of the tracks/channels for the recording and start the actual recording process by tapping the microphone button on the desired target track. A brief pre-count will take place, before the playback of the backing track you have put together and the actual recording starts.