I do my nails differently every time I go to Disney - I've done some crazy designs, especially for Christmas! It's really easy to do it yourself, if you get a regular mani/pedi and want to add a Mickey head on your own.
Wipe the brush of almost all the polish, except for a small bead at the end of the brush. Touch the bead of polish to your nail (not the brush, it may not be circular), and it will create a small round dot. Larger bead, larger dot. Add two smaller ones for the ears, and you're set! You can also use a dotting tool, but if you don't have one, it's easy to do without one.
You can practice on a piece of plastic, to make sure you get the circular dots. If you want to get fancy, you can add a jewel in the middle of the head. Take a toothpick, get it wet, and pick up one jewel. If you put it on while the polish is still wet, it will create a circle of color around the jewel. If it's dry, you can either use nail glue or clear polish to set it. Add a coat or two of clear polish to set it, and you're done!