Don't be too serious about it. This is supposed to be fun!
Step 1: Gather Your Supplies
For the shirt itself, you want as plain of a shirt as possible. The less ribbing/texture the shirt itself has the better. I chose a black "perfect tee" from Old Navy since it's thin enough to wear to an indoor party (remember, the heater will likely be going) and was on sale for $5.
For the actual decorations, the more glittery or stereotypically Christmas the better. Basically, if you can attach it, grab it. I used a lot of glitter glue and self-adhesive felt stickers. I also had stick-on rhinestones, jingle bells, and pompoms.