When laying valley membrane by yourself, gravity can be like a frenemy.
To keep the membrane where he wants it, Ben Bogie tacks it with a roofing nail near the center of the sheet. This holds it centered in the valley while he sticks it in place.
He pulls the paper flat and slow, letting gravity stick the membrane in place as he peels.
Now he pulls the nails and peels the other half.
With the membrane where it needs to be, he deals with those three little holes.
The triangular patch is positioned to so that any water that gets beneath the metal roof and runs into the valley, is directed back into the valley.
Now he can cover that membrane with valley metal