Follow the instructions here to install an open metal valley. The valley flashing must be installed after the underlayment and before the shingles.
If you will install an open metal valley, the valley flashing must be installed after the underlayment and before the shingles; follow the instructions here. If you will be installing a woven or closed-cut valley, there is no need for valley flashing. However, a strip of WSU is highly recommended.
Roofers sometimes make their own valley flashings out of sheet metal, but you are better off buying ready-made valley flashing. The type with a center spine (or ridge) works best because it channels water more efficiently and permits expansion and contraction of the metal. Valley flashing with nailing clips prevents puncturing of the flashing, but you can also nail the flashing directly if you do so carefully.
The drip edge along the rake is also installed at this time. In most cases, you will use the same type of drip edge as you used for the eave.
1 to 2 hours to cut and install flashings for a valley and rake ends
Tin snips, tape measure, hammer, chalkline
Measuring and cutting metal, driving nails
Apply the underlayment, preferably with a strip of WSU along the valley.
Valley flashing, roofing nails
Set the first piece of valley flashing in place, check its alignment, and make sure both sides lie flat on the roof. Drive nails next to the flashing so the nailheads capture the flashing but do not poke holes in it. (If your flashing has nailing clips, drive nails through the clips every foot or so on each side.)
Where a valley meets a ridge, bend the flashing over so it laps onto the other side by at least 3 inches. You will need to cut the spine first. You may choose to apply a bead of roofing cement along the side of the flashing at this time; applying it later takes a bit more time but is less messy.