When a room in the Middle Ages was panelled with big areas of linenfold, my guess is that carpenters used simple moulding planes to run long lengths of the fold into prepared boards. Then they'd cut these boards into appropriate lengths and hand them over to the woodcarvers to finish off the ends. The carvers would get quicker and quicker as they repeated the various patterns and made visual sense of the profiled panels.
We don't know that this is exactly how the workshop would have been set up but dividing the tasks if not the workers like this would be a rational and most efficient way of working - and these were rational and efficient craftsmen.
I'll be using a router to start the work off but turn to my gouges for the actual shaping of the linenfold. One word to remember: symmetry.