Put a roll of felt down the valleys. Then roll out the felt on the rest of the roof, lapping over the valley. Put your metal down. Use W valley if you are going to leave it exposed to catch the water. Use flat metal with another roll of felt over it if you are going to lap the shingles across the valley and create a shingle valley which is being done more and more around here.
I'm with Steve on the 30# felt. 99% of the roofers use 15# felt but if water gets under your shingles, it is comforting (and overkill on my part) to know there is a heavier weight paper to catch the water to keep it off your sheeting(on my house anyway).