I started by covering the back and sides of my frame with white poster board. You can make errors here and not worry too much as the front cover printout will wrap around the sides and hide most imperfections.
I used white glue on the tops of all the cardboard tubes. Cover as many tops as possible with a small bead of glue. If a small amount of glue falls inside the tubes it's not a big deal. Then, run a thick bead of glue along the outside frame.
Carefully line up the printout markers (as seen in Step 4 - mine were the dead trees) with the corners of the box and gently press in place. Since cut cardboard tubes had some variation the poster did not sit perfectly flat. I placed a few heavy towels over the cover for weight and let glue dry.
The corners were cut according to our 45 degree markings (as designed in Step 4). Theses edges are then glued to the sides.