Honestly, the easiest and best way to do it is to start by just writing about your own experiences with Christmas. Share a funny story that happened to you, or something that really touched you.
Then once you have the foundation in truth, add some fictional elements to "spice" it up. I find it's SO much easier to write when the story is already based on something true.
And besides, there's a reason we're so touched by movies that are "Based on a true story" - those stories really connect with us as humans.
Similarly, when you write from a place of truth, you also create that opportunity to connect more deeply with others.
Another approach you can try is from the movie "Finding Forrester." Instead of writing your own story, first start typing a story that you find interesting... and keep typing until new words come out of your brain and the story becomes your own. Then you can just go edit / rewrite the beginning of the story to make it totally unique.
If you need ideas on where to get Christmas stories to start writing from, or just for inspiration then there's a ton of them (both true and fictional) here:
http://www.Best-Christmas-Stories.com