The stitch you are looking for is called "Diagonal Box Stitch".  You can find it on the website below.  I love this stitch and use it for manyy of my afghans designs.  I use my grid (800) 'Paint' program on my computer to work my designs and colors.  You're going to love this stitch.  It works up rather quickly.  I would suggest you make a sample first to get used to the decrease process. I made my sample using 12 blocks.  This tutorial uses two colors so you can see the stitch changes better.  This is the site I used to learn the stitch.

www.crochetcabana.com/stitches/diag-box-st.htm

Garnie