I'm confused, that's my normal state, but are you going to have an open valley or a shingled valley? Are you using shakes or shingles?
If this is your first time, an open metal valley is probably easier. A W valley would be best, it has an inverted V in center, not only does that alleviate water running across valley and under shingles, it helps you center it in valley. Here's a pic and some more instructions on installing. Note that shakes and shingles need different widths of valley and under flashing.
Measure twice, cut once.
Look at the nail, not the hammer. Watch the fence, not the blade.
If you hook your thumb over your belt you won't hit it with the hammer or leave it layin on the saw table.