To keep your cushions looking lovely and to prolong their life they should be plumped up daily.
Do not thump them as this will break the spines of the feathers. Instead hold the cushion by its corners and drop it onto a hard surface from about 12". Turn the cushion around and repeat the action from the opposite side. Then do the same with the remaining 2 sides. The cushion will then magically puff up.
Most of the textiles I use are washed before making them into cushions. The exception being the Kuba Cloth which I have never washed so cannot advise how you would do this.
I don't recommend washing cushion covers but should the need arise, I suggest either a gentle handwash at 30º or dry cleaning. Then dry flat.