After I have all the rows sewn, I lay out the rows and in the order that they go together, I press the seams of one row to one side and the seams for the next row to the other side. That way, when you sew the rows together, the seams will nest and make the corner matches easier.
When I first started, I did not want to hear (or believe) that the devil is in the preparation. Some of my initial trials were done rather haphazardly and I paid for it EVERY STEP OF THE WAY. The more precise the prep work is, the easier the assembly is. Take it from someone who fought that concept hard!