Move the tip of the tongue behind your teeth without touching the front teeth. Make sure that the whole tongue is suctioned up against the hard and soft palate, creating an inter-oral vacuum. Hold it as long as you can. Get used to it, it will become natural overtime.
Breath through your nose all day, awake and during sleep. If you struggle closing your mouth during sleep, consider mouth taping. Lips together, Teeth in light contact or slightly apart and Tongue on the roof of the mouth. The tip of the tongue should be placed just behind the front teeth on the incisive papilla. Use the suction hold method to easily stick your tongue to the roof of the mouth. Consider maintaining a suction hold even while doing sports.

Lower body fat to less than 15% (male) or 21% (female).
Soft Chewing: chewing on regular gum (bolus or small) for long duration.
Hard Chewing: chewing on Mastic gum (bolus or small), etc.
Chew using your back teeth. Chew evenly on both sides. Chew your food slowly, steadily and properly 32 times or till it become mushy before swallowing. Closed mouth chewing. Consider taking a day or two days break every week from hard chewing and prolonged soft chewing. Consider massaging or stretching your chewing muscles by yawning or opening mouth widely after hard chewing or prolonged soft chewing.