Wash your t-shirt first (this helps the fabric paint stay on later).
Now you need to make your stencil! Use an image from online and print it onto a piece of paper, then trace it onto your cardboard, OR hand draw onto your piece of cardboard.
The design should measure around 4 by 2.5 inches, but you can vary this depending on what you want.
When you've drawn your design, cut your stencil out with scissors (and join it back up with sticky tape if you had to cut into the edges).
Place the stencil onto your shirt, making sure it's parallel to the top and bottom of the shirt - so that it's straight.
Fill the stencil in thickly with your fabric paint. Once it's filled in, remove the stencil carefully and fix up any edges with your paintbrush. If you found antlers too fiddly to put in the stencil, but want little reindeers like me, you can also hand paint your antlers on at this point!
Paint your deers onto your shirt (you can use the images in below steps as a reference).
When you flip the stencil over to do the ones facing backwards, make sure to clean your stencil with some tissue paper first!
When you're done, let it dry for a few hours. Then, run over each deer with a hot iron for 1-2 minutes. This should make it washing-machine safe (as long as you started with a decent fabric paint).
You're done! Enjoy your new t-shirt!