Cut both fabrics in 3 1/2 inch squares.
Cut the darker fabric (or the "outside" fabric) in half diagonally.
Then place one of the darker triangles on top of the lighter square, right sides together.
(Isn't the intarsia on my treadle beautiful...)
Now line up the edges and sew a seam 1/4 inch from the edge. It saves a lot of time to chain sew all the blocks at the same time, just keep feeding them under the foot one after another.
Cut the blocks apart and press them open, first set the seam by ironing them just like they are sewn.
Then press the seam open, keeping the seam allowance towards the darker fabric.
Now sew and press the opposing triangle.
...and the other two triangles.
Now it's time to trim your blocks down to size. I use Wendy Mathsons rulers for her Storm at Sea quilts. If you want to do it with regular rulers, trim them down to 4 3/4 inches making sure the square is centered (use the diagonal lines on your ruler).