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.
Step 2: Sketch Your Reindeer
Sketch the reindeer out on your shirt with chalk. He doesn't have to be perfect, and you don't have to worry with details like the eyes. All you need is an abstract silhouette that is vaguely deer-like. Once you have it sketched out, paint a base layer of white paint. This will help the browns stand out.
Step 3: Add Your Tacky Pieces
I used hot glue to attach the jingle bells & pompoms.
Step 4: Paint Your Reindeer
To make the antlers, I added a bit of white to the brown paint. Then I added just a tiny drop of black paint to the brown to make the hooves.
Depending on how thick your paint goes on, you may need to do a second coat. Make sure to save some of each color that you use just in case you need that second coat.
You can pair your finished shirt with plaid pants, Santa shorts, reindeer antlers, etc. to add even more flair.