Confusion about Shed Roof Cabin Framing

[DavidMSmith]

I am planning on building a 20'x12' shed roof cabin and am having some confusion when modeling the framing in SketchUp, specifically regarding the side walls (scratch walls) and how the rafters sit on the top slab. The front wall is 12 feet high, while the back wall is 8 feet, using 2x6 lumber. 

This approach doesn't seem right and I am unsure how to properly frame this section to ensure proper stability and support for the roof. 

Any guidance on the correct framing technique would be greatly appreciated!

[MountainDon]

.......This approach doesn't seem right and I am unsure how to properly frame this section to ensure proper stability and support for the roof.

I don't fully understand what part you are questioning. The supporting wall design or the roof rafter design?  In case it will help here's a drawing of the rafter to wall joints.
 

[MountainDon]

The AWC calculator may be handy for sizing the rafters. 
https://awc.org/calculators/span-options-calculator-for-wood-joists-and-rafters/

There is a tutorial and versions for Android and iOS