Please note that I use twine as I find that it sticks best to the fabric strip while I work with the fabric but feel free to use any kind of thin cording.

Fold your fabric strip in half, wrong side out.

Press well with your iron on steam to form a sharp edge on the fold.

Place the twine cord right on the fold, leaving one end to peak out just a bit. The other end should hang outside the strip at least 1".

Pin the twine cord to the top of the tube, on the end where the twine peaks out just a bit. A small safety pin would work too.

Attach the zipper foot to your sewing machine.

Sew perpendicularly to the twine cording, thus trapping the cording. Use a small stitch to make sure the cording is tightly secured inside the fabric strip.

Place the folded strip under the foot lengthwise, with the twine trapped in the fold. Hold the twine so that it lays flat and straight inside the fabric strip. The tip of the zipper foot should press the twine against the fold so there's no more room between the cording and the fabric.

Sew down along the twine cord to the other end of the fabric strip as far as possible from the open edge of the strip. Do your best to keep the needle from catching the twine. Seam allowance will vary depending on how wide or narrow you want your spaghetti straps.

Trim the strip diagonally first then trim the seam where the twine is peaking out. From this point on, the small tail of the twine will not peak out anymore but I'll just refer to it as if it still does, for ease of reference.

Continue trimming the raw edge of the seam along the twine cording as close to the  seam as you can.

To beginning turning the wrong sides inside, grab one end of the twine (with the long tail) with one hand and with the other hand grab the opposite end (the one with the small twine tail).

Pull the long tail just a little bit while holding the fabric at the opposite end (the one with the small tail). The fabric will tend to gather at the small tail end. You are aiming for pushing the small tail twine inside the fabric tube.

This will prove to be a bit difficult with just this method alone so you need to push the small tail in with the help of a blunt tool such as a bamboo skewer.

While pushing the tail with the skewer you need to force the fabric a little to roll over the skewer, using your nails.

Once the tail has passed the first 1/4"-1/2" of the tube, turning the straps out is nothing but a breeze. Keep pulling the long tail with one hand while pulling the fabric with the other hand on the opposite end.

When the entire spaghetti strap is completely turned inside out, snip the sewn end to remove the twine cording.

Using your fingers, smooth out the seam.

You can leave the straps as they are or press them into flat straps using your iron on steam