You should write how you feel about the person getting this card. The quality of the writing is less important than the fact that the thoughts are coming from you.
It is always really nice to write a happy remembrance of something the other person did, a time spent together, a funny or beautiful moment, or a quality you appreciate about the other person.
A reference to something you've shared with the recipient this past year or that you both will look forward to. It's nice to also add a photo of the both of you.
