What does the care label say? Are the panels washable or do they need to be dry cleaned? If washable, go ahead and follow directions. I know sewers that take their fabric to the cleaners before sewing it if it's important that it be done. I would go ahead and sew them and clean them later, IMHO.
It sounds good. Could you translate to USA, please? I have no idea what "stork margarine" is or "caster sugar." There must be gram-to-cups tools somewhere. (And I am too lazy to do all the internet searching!!)