Make up the four valley boards out of 50mm x 25mm (1" x 2") stock slightly longer than the valley, as the ends can be cut off later. Cut two bevels along each valley board as shown in the illustration below.
This is best done on a bench saw with the blade tilted 30 degrees. With a certain amount of skill, it can also be done with a circular power saw by holding the member in a vice type workbench.
The top of the valley board will need marking and cutting to fit snuggly into the corners of the ridge capping. This may need a bit of trial and error.
Trimming the ends of the ridge and valley boards
With a sharp handsaw cut off the ends of the ridge capping and the valley boards so they are flush (even) with the plywood roof panels.