In order to fit the inside paper layer, I laid out the back and side chipboard pieces and traced around with a pencil. Once cut out, I used a scoring tool to mark the inside folds. An alternative to this fiddly method is to cut individual back and side pieces, but I like the continuous design (and I'm not a fan of seams).
I used Mod Podge to adhere paper to the back piece first, then cut out the notches for the roof boards.
The sides aren't added yet, so I used the table edge to fake the 90 degree angle in order to mark the notches for the sides. You'll see I originally cut more than I needed for the walls, knowing it would be trimmed later.
Here I have the bottom inside layered with paper, and I've cut out the roof inside piece. When assembling, it's best to glue one side to the back, then the bottom, then the other side. Save the roof for last, since it will have to fit into the front pieces top notches.
I used a paper cutter to reduce the tree width a bit. I inked all the pieces dark brown, then embossed for texture. Not pictured, but here I put spots of glue and snow on the brach tips and allowed to dry.
I glued the small branches toward the back of the box, then added the full tree in front for dimension.
This scene needed icicles since it almost looks like we're peering out a window. I cut the icicles apart and painted white, then used glitter glue on top.
I outlined the front piece on coordinating paper, cut, and used Mod Podge to adhere to the board. I sanded down the rough cuts in the center and along the bottom. You can also see I painted the inside bottom black,
Add the icicles to a corner with white glue.
Carefully glued the front to the rest of the box. Now it's time to add the roof. You can see here I ended up cutting the single roof paper into two pieces. It was easier to handle.
I used two snowflakes for a star -- painted gold then layered with gold glitter glue.
And that's the last in-progress photo! The roof is a single cut of paper, scored in the center. The back was measured and cut in the early stages and set aside. The feet and inside bottom are all black paint.