1. Right click cell and select format cells
2. In the border menu set a diagonal line
3. Type 2 words in the cell with some space between
    them so each word is on opposites sides of the cell
4. Highlight the word that is supposed to be on top and select the small
    arrow in the lower right corner of the "Font" menu
5. Make the word a superscript if it's supposed to be on the top side of the
    diagonal line and a subscript if it's supposed to be on the bottom side.