You will need
1 metre of Organza
The vase cover is effectively a tube of Organza. I cut the organza to a 1 metre by 1 metre square. I decided to use either a flat fell seam or French seam. Both start by pinning the fabric WRONG sides together. Sew these sides together.
Now fold the tube half within itself, so at one end you have a fold and at the other end of the tube you have both hems. place an elastic band around this tube and slide it over your jar or vase.