They don't shrink. We washed ours a couple times now (liveaboards, so get lots of use). 30° machine wash. Also re-dyed them once in the washing machine.
The backrests are easy, two zippered compartments with foam, soft side facing the human. Only problem is, they didn't do the neatest job of cleaning up the fabric inside and lots of strings will come out. Bit messy, can be trimmed down a bit with scissors though.
The seat bottoms are more difficult, as there is a batten inside. This batten is held by loops of white string and a plastic bar. A knitting needle is extremely helpful in taking them out (hook the white string, pull it off the brown plastic bar, then remove the plastic bar). Also in putting them back in, where you must poke the white string through the foam the other way and re-insert the plastic bar to tension the batten. The batten itself is glued into the fabric (in a pocket) and this was a huge pain to remove. Gentle warming with a hairdryer softened the glue enough to wrestle the battens out, but several of them were already broken (before removal) and we just left them out on the saloon cushions, which makes putting the cushion back together much easier, but it no longer has that indentation near the front. Would be easy to replace the battens, it's just a plastic stick.
If the zippers are seized, use some fast release penetrant and patience to work them lose and after washing a bit of silicone oil to keep them from seizing up again.
Putting the foam back in before the covers are completely dry keeps them from getting wrinkles.