When working a valley I lay 2 shingle side by side at a time. For some reason if I try and go up the valley by laying the valley shingle first the shingle end up being slightly cocked. So I lay the shingle adjacent first so I know it’s straight, then I butt my valley shingle to it. In this manner you run up the valley in aprox a 45 degree angle. Depending on you valley you will probably have to “jump back” your seam every so often as it will drift in too close to the valley.
“Do you layout the valley and then start laying shingles at the rake? You would then just trim the shingles when you hit the valley shingles.”
Sooo run up the valley 2 by 2 (like Noah) in aprox a 45 degree angle. Then as your seam between your 2 shingles starts to drift into your valley too much, slide your next 2 shingles back out of the valley a bit (much like a typewriter) so they are the aprox distance into the valley as the very first 2 shingles you laid. Rinse, repeat.
It’s easier and faster to cut a rake so I start in the valley. The exception are shingles that need to be laid out straight up the roof like 3-tabs. In that case I go up the rake but I will run 2 shingles at a time up the rake in the same method I use to run up a valley. IMO it just keeps them straighter.
“Do you shingle from the rake and just shingle over the valley with the manufacturer’s offset?”
No. FYI the manufacturers offset is in most cases a guideline. I don’t pull out my tape measure to get exactly 6-1/2" every shingle I just go by eye and shoot for 6-8" offset, which is plenty. Architectural shingles are very forgiving.