I have often mentioned that a well-written letter from the heart is a great gift for almost any wife. This is not limited to those whose love language is words of affirmation; most women will feel deeply loved by a Christmas Love Letter.
The goal is to tell her how wonderful you think she is. Thank her for the ways she has blessed you this year, and admire her for the things she has done for others. Give her credit for making an effort even when the results may have fallen short of what she wanted. Tell her she is the only one for you, and being with her is better than any gift she could buy and wrap. Tell her you are looking forward to another year with her and to growing old together. Tell her she deserves better from you, and you are working on that.
Unless you have a great sex life and she is very much into it, I would avoid the topic, or make one passing reference. You want to praise and thank her, not challenge, or hint at what she could do better. Make it all about how great you think she is.
If you have never done this, try it. Spend some serious time on it; don’t just bang it out in one sitting. Unless you have great penmanship, I would suggest you print it out. A page is great, much longer is too much, and half a page is too short. Give it to her when she will have time to read it and cry a bit. Leave it on her pillow, or if you live alone put it by the coffee so she will see it in the morning. An unsealed envelope with her name handwritten on the front is a nice touch.