When the old material on seat cushions starts to break up, as it did on this one from the 308, the only solution is to replace it. Today's helpful hint: Black duct tape will not work! This artificial leather type of material went by several trade names, such as Fabrikoid or Pantasote.
It's held to the seat frame with hundreds of small carpet tacks, each of which has to be removed. I'm not sure whether the frames date back to the car's construction. This frame had been on its second covering, and I installed the third. That may be approaching the limit to how many different sets of tacks you can drive into the wood. The modern material is less prone to ripping, though, so I can space the tacks somewhat farther apart. 
I suppose one could instead use a power stapler, but I prefer to stay with the original system, however time-consuming.When done, it looks like this. For our level of service, this should last indefinitely.