* Sew the seam with the sewing machine.
* Overlock each seam allowance separately.
* Steam the seam open.
* Tack the seam allowances down with a catch stitch.
A video about this subject: https://www.youtube.com/watch?time_continue=41&v=d783OdG_7fs
Here’s a link the tells you how to lengthen sleeves of a finished garment.