Select the required cells. Then from the Format menu select cells. Then
select the Border Tab. Choose the line style you require in the box on the
right and then click just below and to the left of the work "Text" in the
box on the left. Click OK.
1. Format > Cells > Patterns and then select one of the diagonal options
from the Patterns drop down.
2. Display the Drawing Toolbar and add the lines manually using the Line
function (click and drag).