For the first week, simply notice your breathing pattern throughout the day. Most mouth breathers are completely unconscious of the habit. After meals, during work, while exercising - capture when you are naturally mouth breathing.
Once aware, begin consciously transitioning to nasal breathing for increasing durations. Start with five minutes of conscious nasal breathing, multiple times per day. Extend to 10 minutes, then 20 minutes as this becomes easier.
Breathing Exercise: Start by breathing in through your nose for one second, hold your breath for one second, slowly breathe all the air out through your nose as long as you need, and then hold your breath for as long as possible. When you can’t hold it anymore, breathe in through your nose for one second, hold for one second, and repeat the entire cycle. Do this for three to five minutes whenever you can. This improves CO2 tolerance, nitric oxide production, and resets your nasal breathing.