Need wood size identification (or suggestions) on barn door picture
If you're doing the herringbone inside you won't use 1x8. You'll run 1x6 all the way out. This design probably ends up the thickness of a two-by (1-1/2"), so your front frame probably shouldn't be two-by. I'd use the same 1x6 for everything (tongue-and-groove for the herringbone, square boards for the frame). If you don't use tongue-and-groove for the herringbone you'd need to apply a backer of something like plywood. It'll tend to crack up and be unstable otherwise. 

You'll need to decide whether you want exposed fasteners (for a truly rustic look) or hidden fasteners (for a cleaner and more modern look). If the former, you probably want conventional nails. Screws would destroy the illusion. A good quality wood glue carries most of the load. 

Here's one simplified plan:


Build your front frame using dowel or biscuit joints, pocket screws, or whatever you have available or meets your skill set. Square the frame by measuring from opposite corners. Skew as necessary to equalize those dimensions.
Flip the frame over and draw a 45 degree diagonal line as a reference. Lay some boards out and shift the line as needed to vertically center the angle joint rows.
Assemble the herringbone. Use appropriate nails or screws to fasten the ends to the frame, and use wood glue on the tongue-and-groove joints. 
Sand edges and corners, or use a router to bullnose. 
Stain and seal or paint.