It’s unconventional, but building the roof upside down on the platform has advantages. First, you avoid a lot of ladder work. And second, you can nail the soffit material to the overhangs easily without having to work overhead and in cramped spaces. You do have to take a little extra care to make sure the framing is square and the perimeter 2x4s are perfectly straight before nailing on the soffit, though. Start by cutting out the rafters using the pattern in Figure D as a guide.
Build the framework for all four walls. To account for the fact that the front and back walls are different from each other (due to the doorframe in the front) and the side walls must both be sloped (to prevent rain from collecting on the roof), each of these will have to be tackled somewhat differently. It’s easiest to construct the back first, the front second, and the two sides last, as shown in the numbered image below. See How to Frame a Wall for more information before you read the instructions below.