Sew the med swatches to Border A and cut the border lengthwise into two borders.
Sew the two long borders onto the remaining two sides.
Trim the borders to 3/8" from the seam on all 4 sides.
pay attention to the squareness of your 9 patch. Corners can get pointed on the corners if your seam is not straight. Go back and straighten a seam if you need to to get a good square 9 patch. Measure every time you sew a seam to make sure your measurements are accurate. If a seam is off even a 1/16" it will skew your block. It isn't hard to go back and sew another seam along the existing seam to straighten it. But at the end is not the time to figure out something is cockeyed.
At this point your 9 patch should measure 3/4" plus 1/16" from seam to seam(the blocks in it not the whole 9 patch)