B.K.
K. -
I ahd this problem as well! I tried placing a small feather on my daughter's lip and told her to blow air from her nose to make it fly away. We played this game for awhile and she had a lot of fun with it. This got her used to pushing air through her nose with her mouth closed. Then I would put a kleenex up to her nose and tell her to blow air like she did with the feather. It seemed to work.
Good luck!
B.