I use lead-coated copper, custom fabricated at a local shop, for all my flashings because it will perform as long as the wood roof — which, installed over a ventilating substrate like Cedar Breather, can last for more than 50 years.
I install the first valley shingle of the starter course, nailing about 1 inch above the exposure line and 6 inches out of the valley. I use 1 3/4-inch stainless steel ring-shank nails exclusively, with the gun pressure adjusted to set the heads flush with the surface. The opposing shingle goes on next, followed by a 12-by-12-inch square step flashing, bent corner-to-corner, with its lower tip held to the butt line and nailed at the two outside corners.
Rather than beating a piece of lead into a lumpy shape over the intersection, I had a piece of custom flashing made.