mix a dice sized amount of white with half a dice sized amount of black. this should leave you with extra clay at the end... i just like being careful so i don't have to mix more later.
also - if you mix this and it is the same color as your cube body, you are going to want to add white.
then you should have two balls of clay, one dark gray (the larger one) and one light gray (the smaller one).
the next step is then to mold the larger ball of dark gray clay into a cube. just use your fingers and play around with it until the sides are 