Those cushions were not meant to be frequently removed and reinstalled. I am not sure if your seat backs are bolsters ot full cushions but if bolster type they are installed just like the side (deck) bolsters. There are lots of posting about bolsters but in general, they are hung on brackets mounted to the sides of the gunnels. In addition they will be screwed to the side with several screws that are driven through the drain grommets on the bolster bottom. It is often hard to find (feel) for those screws and more importantly very hard to reuse the same holes when you remount the bolsters. Some folks just don't use the screws and that may be OK or maybe the bolster will pop off. More often than not, when reinstalling bolsters, new pilot holes are made for the screws.