Blank dice

Here is a probability brain teaser that I remember from a math for elementary ed class I tutored.

Puzzle

When you roll two six sided fair dice and add the dots displayed, you can get any number from 2 to 12. However, some sums are more likely to appear than others. Now suppose you have two six sided fair dice, with no dots on any of the faces (they are blank). Armed with only a Sharpe and your puzzle solving prowess, draw dots on the dice so that each sum which can appear will appear with equal probability.