Actually those sort of fold up in the leather, then the whole skin is stretched and those plastic sheathes slip into the runner along the inside edge of the seat frame. New material might only have enough give for one fold. Easiest way to stretch and slip is to roll it in the material then lay across the cushion, allowing your body weight to compress the pad and springs.
You'll see what I mean when you take the old skins off.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.