As Kevin suggests just a few inches at a time. When you put them back on put a thin plastic on them (such as dry cleaning bags) will help a great deal to get them back on. To get them off if without plastic on them it helps if you have another party compress the foam while you get the covers off.
I just removed mine last week by peeling them. I was afraid of breaking the small zipper. Getting them back on i had to turn the covers inside out and go backwards...slowly inch by inch. Time consuming but no damage for me.
I removed my covers and put them in the tub with Wisk...and shook and rubbed them all around...omg talk about dirty water!!!!!....sheesh. It took about 10 rinses for the water to run clear.