Flip the template so that the printed side faces up and sew the seam between sections 1 and 2, sewing exactly on the line (Fig 4 first below).
Extend your sewing line a few stitches beyond each end of the line (Fig 5 below)
Note: This rule of sewing a few stitches over at each end applies when the seam sits within the block and does not reach the edge of the block. If, however, the seam touches the edge of the block, continue the seam until you reach the dashed line of the seam allowance. You may need to move the pins away from the seam to make it easier to sew this line or even transfer them to the front of the template.