Here is a link how to connect your device to Bluetooth:

In iReal Pro version 8.3:
In case the playback position marker is lagging, you might need to adjust it.
Tap on:
More -> scroll down to Advanced audio settings -> Bluetooth lag adjustment

It is caused by a delay within the device sending the audio output through Bluetooth. 
By default, it is set to 500ms but different devices may need a different setting.