I just followed the assembly instructions provided with the structure.
Unfortunately, the structure kit I received had some warped studs. This was a big dissapointment, and I could have probably gone to my local lumber yard and gotten new, straight ones, but I decided to use clamps to hold the wall frame down on the assembled floor and then nail the panels to the frame.
I just followed the assembly instructions for the structure and actually needed a bit of help with the raising of the walls to assure that they were level and didn't fall over before nailing them.
This was the whole reason that I constructed this shed as opposed to others I found. The walls, one up, create a fully structurally sound building, with the room just something that is added to the top of the 4 walls. As you can see, the walls are up, but now the structure to hold on the roof.
I simply added additional 2x3's to the top of the wall studs and then a 2x4 (later) to support the rail for the wheel tracks.