|Spaghetti straps can be made quickly and easy with a serger. First, set up for a Rolled Hem. Sew a chain 5" to 10" longer than the length of the strap. Bring the chain around to the left of the presser foot and to the front of the machine. Take the fabric strip for your strap and fold it right sides together, around the thread chain with the fold to the left. Begin to sew, holding the chain next to the fold of the fabric. The cutting blades will trim away the excess fabric as you sew. Be careful not to cut the chain. When the sewing is complete, pull the thread chain to turn the strap to the right side.
This is how I was taught in a workshop this week, very nifty. I have copied the text for this tip from the Janome site, as it explains it much better than me.