... to a row of vertical sashings. Yes, all vertical rows get the pink paper. Since I'm not working with the corner squares yet, I take one of the letters and pin it to the top of the row square. All the other 3 squares in this row will go to the back of this top square and this tells me what order to sew the squares together.
Now, what do I do with the 2nd. letter paper? It gets pinned to my design wall to tell me that the row A that I just removed will get returned to this same spot after it has been sewn.
I have this obsession with keeping my squares to be sewn on the straight of the grain (sog). This dates back to my days of sewing clothes and nothing makes clothes fit better than cutting out the pattern on the straight of the grain. Same for sewing for quilting (except where the bias comes into play). In this case, for the vertical rows, arrange your squares with the straight of grain (sog) going up and down (or north and south).
For row D, this is the order which they were on the design wall. When you take your D pile, place the bottom square from the pile close to you and then place the next one from the bottom of the pile against that one and so on until you have the top square with the paper on it at the top of the row. Sew squares 1 and 2 together and then squares 3 and 4 together.
At this point, clip the threads holding row D (see the two sections above my scissors?).
Now, sew sections 1 & 2 to 3 & 4 as shown below.
Row C (photo below) shows the two sections that need to be sewn together.