It sounds like you're in need of a detailed description or an example of how you spent the Christmas holidays with a friend. There are many things you could include in this type of assignment...
Since the person you're writing to is your brother, you will probably have that freedom to be honest, open and relaxed. So the writing style would be pretty informal.
The first thing to do when writing a letter is to address the person, for example, "Dear Andrew", followed by an opening statement. Since it is an informal letter to your brother, you can go straight into talking about your experience without having to worry too much about formalities.
So you could say, "this year I spent Christmas with _ " and then continue to explain all the things you two experienced together. This could include the family members you were introduced to, what you had for Christmas dinner and how you felt during your time there.
I hope this helps.