Double Welt- Used in the place of Gimp, Hide-em Gimp and striping &
Decorative nails. The nice part of Double welt is you already have
everything, the fabric you're using now and you don't have to worry
about a color match, as it's the same
I've got a picture here for you and I'll do my best to
explain it.

The sewing machine presser foot is a single welt foot.
#1 Sew a welt into the fabric (make sure the fabric is a least 2" wide)
on the left side.
#2 Now with the one welt cord in the fabric turn it end for end.
#3 place the another welt cord in the remaining fabric (this will be
face down towards the table of the sewing machine.
#4 Now place this under your presser foot so the needle lines up with
the other thread from before or right between the two welt cords.
#5 Pull on the fabric flap that will be underneath hold it snug, to hold
the welt cord in towards the needle and sew.
You'll have two thread stitches showing, Now cut off the excess fabric
and you should have a very nice double welt cord.
Normally it is attached by hot glue, you can also use a
slow drying
glue, but then you must hold the double welt in place with tacks or
staples until it dries and then of course remove them.
Merv
Merv's Upholstery & Work at Home Training Videos
Http://www.mervstrainingvideos.com