Let's use the back of a halter dress for an example:
DOUBLE the measurements of the width of the fabric for the back.
Finish all your edges the way you want.
Take a spool of elastic thread, and wind your bobbin on the machine just as you normally would.
Place the bobbin in it's place and use REGULAR thread for the front side of fabric.
Holding the fabric taught, with your machine on the loosest tension and the stitch set to wide and long, sew across the fabric front side up.
The elastic stitch should be on the inside of your garment.
I use the presser foot as my guide as to how wide to space my rows.
Sew all your rows, still holding taught. OH DO NOT LOCK YOUR STITCHES IN THE BEGINNING OR AT THE END OF EACH ROW.
When you are done, pull each stitch tighter to gather a bit more.
Then pull the reg. thread stitch through to the inside of garment so it is next to the elastic understitch, and tie them together to lock. Do that on each side of the rows.
That should be it.
I hope I explained it well enough!!!!!!! LOL
I am not that good at teaching! LOL