Tilt your head back, with your ears submerged
One to practice in a pool:
Instinctively, most people who find themselves struggling in the water will begin to panic, swim, or thrash about. We’re urging people to ignore this instinct and remember to float: Tilt your head back, with your ears submerged. Relax and try to breathe normally. Move your hands and legs to help you stay afloat if you need to. It’s fine if your legs sink – we all float differently. By doing this, you give yourself the chance to rest and recover your breathing. Once you’ve regained control of your breathing, you can call for help or swim to safety.
From: RNLI urges people to Float to Live.
Why ears underwater? The "science behind..." video explains:
[...] telling people to get their ears in the water just meant they put their head back a bit which lifted the chin which helps keep the airway clear of the water.