1. If you want to give it an old paper effect get a tea bag and put it in hot water, just like making tea, leave the teabag in for about 30+ seconds then drag it along the paper.
2. Dry the paper with a hair drier.
3. When it's dry you can print your letter onto it or hand write it.
4. Use a font that you think Santa would use.
5. You can roll it up and wrap the ribbon round it, then leave it under the tree with his/her presents.
Here is a link to some Santa Fonts - http://www.fontspace.com/category/santa