Cut a piece of 1/2'' wide elastic at 7''. Mark the center and 1 1/2'' from each end.
Pin the elastic on the wrong side of scarf on the gathering line, lining up the center marks and leaving the 1 1/2'' ends hanging over.
Carefully put the scarf under sewing machine as to not to disturb the gathers. Hold on to the 1 1/2'' end of elastic and stretch out the elastic from the center mark. Backtack and sew a regular stitch while keeping the elastic taught.
Backstitch at the end and you should end up with this.
Trim the ends of elastic, you may use Fraycheck to prevent fraying. And you're done!