Start a corner. Thread your embroidery needle with a 20" length of Sashiko Thread. Tie a small knot at  one end. Poke the needle in between the two layers of fabric and pull it out about 1/2" away.

Pull the thread fairly hard until the knot gets pulled into the middle of the two fabrics. Don't pull it so hard that you pull the knot out entirely.

Sew down the long length of the scarf using large running stitches, anywhere from 1/4"- 1" long. Don't pull your stitches tight. They should be relaxed and should not pucker the fabric. Take out the safety pins as you come to them.

Make sure to get through both layers. This does not have to be ultra even. The beauty comes from the variations.

When you come to the end of your piece of thread tie a little knot 1/2" from where you last came out. Stick your needle in between the fabric layers 1/2" from where you came out and give a little tug. The knot should disappear in between the layers like it did at the beginning of the row. Snip your thread end.

Quilt the rest of the scarf as follows. Sew each row in the opposite direction as the previous row. This will keep the scarf from puckering up a help it stay flat.

First quilt both edges and the middle.

Then in between these lines.

And once again in between the previous lines... You can keep doing this until it's even more quilted but this seemed like a good place to stop for me!