You will now trim the square into a 8 1/2" block as you will each block you make so that all are 8 1/2" unfinished.
You will place the folded strip on one side of the square raw edges together and the open strip on the back side raw edges together then sew and press open only the non folded sashing.
This photo shows both sashings open where the other square would slip in.
Sashing time... 1 strip 1 1/8" x 8 1/2" for back and 1 strip 1 3/4" x 8 1/2" folded in half and pressed so that right sides are showing and wrong sides are together.
Take your second square and sew it onto the pressed open sashing. Then you will turn it over and press open the folded strip so it now overlaps onto the other square and slip stitch it like you would your binding. You now have two attached squares.
In this photo you can see two squares (different sample) sewn together and the sashing strip is now completed on both sides .