Say I love you. It's as simple as that. Those 3 simple words go a long way, trust me.
When the time is right you'll let her know but if the relationship is still young, wait a while or you'll scare her off for sure and then you'll regret it so be patient and sign her Christmas card with Love, your name and see what happens. So make sure this is what you truly want to tell her. Good luck and Merry Christmas