Cut a strip a little longer than required.
Fold in half lengthwise with right sides facing.
Tie a knot at the end of a piece of string and place the length of string in the fold of the fabric with the knot just showing at the top of the folded fabric. Stitch across the top of the tube to catch the string firmly in the stitching, then stitch the long side seam, taking a ¼ inch seam allowance.
NB Keep the string in the fold and do not catch in the side seam, and keep the free end of the string below the point at which you are stitching.
It may take a little jiggling to get the tube to start turning, but then it is easy to pull the string through the tube, pulling it right side out.
If the string is not as long as the tube you can start pulling it though the tube while you are still stitching. Just stop with the needle down and start pulling the tube right side out to give you more string to work with.