I made a base, since Miis have such big heads that the little feet are not sturdy enough for it to stand on it's own so. You can use whatever colour you like.
Take a fairly large piece of clay and knead the clay for 2 minutes
Roll into a ball and press flat (it should be about 0.5-1cm thick)
Press the feet of the Mii into the base. Make sure that it is secure and the Mii stands up straight.