Here you see the double 2x6 headers over the doors and windows in the front and rear walls. (These are the 14 foot walls and carry the roof load.)
We laid out each wall right on the deck and took measurements from our actual door and window sizes. These we had collected over the years.
We put the top and bottom plates together so we could mark each stud and where the doors and windows go. Then we just laid out the precut studs and pounded the frame together with two 16 penny nails at each stud.
Tilt the wall up, brace it level and then go on to the next wall. Not so tough, really.
When all the walls were up we ran a second 2x4 plate all the way around overlapping the joints in the walls below. This ties everything together.