Sew the squares to the light rectangles and press the seam allowance towards the light side. This makes it so much easier to press later that I don't care about the 'rule' of pressing towards the darker fabric. Join the two strips together as shown
The next step is to draw two sewing lines across the unit. There are three points to check when you line the ruler up: make sure the line goes through the very corner of the block, the place where the seams cross in the square and that the 45 degree line on the ruler runs along the edge of the block. Sometimes you may have to jiggle it a little, but these three checkpoints will make sure you're drawing in the right place.
Stitch along the drawn line; it feeds through more easily if you don't start at the corner (some machines chew the points here) and when you've sewn one, pull the piece out from under the foot, turn it around and sew back down the other line.
I press the piece to set the seams and get it flat before lining the 1/4" mark of the ruler on the seam line and cutting the block into two. Because there is extra allowance built into the dimensions I then trim the other piece, but you could just make one cut down the middle and have wider seam allowances.




















