When the recording screen opens it will say "waiting for signal' on the watch it means that the app is listening for the ultrasonic signal from KardiaBand EKG sensor.
When you begin a recording by pressing a finger against the exposed side of the KardiaBand sensor, with the inside electrode of the sensor against your wrist, it should emit its EKG signal and the watch app should begin recording.
Please read this article to learn how to take a proper EKG with the KardiaBand.