If you're writing a Christmas card to a friend, it's important to make sure your message is heartfelt and genuine. A good place to start is by expressing your appreciation for their friendship and how much you value them in your life. You could also mention any special memories you have shared together, or any experiences that have brought you closer together. Finally, you could end by wishing them a joyous holiday season and expressing your hope that the new year brings them much happiness and success.