Fold your strip of fabric in half along the width and cut (so you have two sections about 25cm x 55cm). Create a 1cm seam along one long edge, right sides together, but stop approximately 7.5cm from the end. Use pinking shears or self bind the raw edges to tidy them up. Press seam (including unstitched part) open.
Now edgestitch around the open end of the seam (on the right side about a presser foot's distance from the edge. Lift the presser foot to stitch across the bottom of the opening and again to go up the other side - like this:
For the lower edge, fold up and press 5mm, then fold 1.5cm and press again. Edgestitch this folded edge on the wrong side, leaving an opening of around 3cm into which you insert your elastic.
Hope all is well in your world