Getting Slipcovers on

ELK

Beautiful slipcovers arrived from Nucamp....Putting them on with plastic bags to help them slide over the cushions...but the cushions stapled (rear bench) to wood are SO hard to get over that both zippers popped on the 2 seat cushions. Before I put the last on (long backrest) I wonder if there is a trick so I don;t pop 3 zippers.....