The reason swimmers should exhale through their nose when their face is in the water is to prevent the feeling of having to gasp for air during the breath. Breathing while swimming should feel as natural as possible, thus some continuous exhale beginning right after you breathe in. If you exhale through your nose while on land, you should also exhale through your nose underwater. I swam competitively for many years and this became natural over time.
One way I've found to coach this is to hum a tune or just make a humming sound with your mouth closed right after you take a breath. When it's time to breath again, you just need a little puffing or popping action from your mouth to clear the way and you are ready to inhale immediately. There should be only a little air left to blow out when you turn your head so you have plenty of time to take in a full breath without feeling rushed. It's also totally fine to breath out through both your nose and mouth, but I mostly blow out through my nose. It makes it easier to control how much I'm exhaling when I only exhale through my nose.
The comments above give good advice on how to start, but you should definitely make the effort to make it a habit.