Swimmer’s snorkel, first…what are they? They’re snorkels that are front mounted as an aid to swimming. It’s attached to the front of your head instead of the side like the ones you see from scuba divers and vacation snorkelers. It creates a streamline shape that actually bends towards the back and straps around your head. 
It’s designed to help improve the swimmers technique. How? Because you don’t have to worry about breathing so that’s one less thing you’d worry about. You can bring undivided attention to other parts of your swimming like the body roll, stroke, vertical forearms and kicks. It’s also a good way to remind yourself to look down when you’re swimming.
Now, how do I breath through them? Exhale through your nose. You are using your nose to blow the air out before you inhale. This technique makes sure you’re not sending breathed out air back into the snorkeling pipe. That means the swimmers snorkel pipe will always be filled with fresh oxygen. You can move air out quicker as well.
Another technique is to exhale through your mouth. The downside to this is you’re bringing excess CO2 into he pipe so you’re mixing it with fresh air to bring a mixture back into your lungs. It makes it difficult to breath compared to exhaling through your nose because you’re inhaling less oxygen and more carbon dioxide.
If you’re still struggling to use the swimmers snorkel, maybe the issue is getting into the water being afraid you might choke if the water was to fall through the tube. If that’s the case, you can add an attachment called the Dry Top. It prevents water from going inside the snorkel while you’re splish splashing through the water.