Step 1: Obtain Materials
A movie theater poster, printed on both sides- in other words, the back is printed with a mirror-image of the front of the poster to allow light to pass through.
A poster frame to fit said poster (I couldn't find a frame to fit my poster (40x28), so I had to trim it.
A flourescent lamp assembly with switch (I swiped mine from an under-shelf lamp from Wal*Mart)
Step 2: Trim Poster to Fit
Place the cardboard over your poster, covering up the area you'd like framed.
Trace around the cardboard with Sharpie.
Cut from the poster anything outside the traced area, roll up the poster, and put it aside.
Step 3: Piece Together the Light Box Part I
With a utility knife, remove the area within the margin.
Set aside the 2 in. wide cardboard frame and measure the remaining area. In my case, the resulting rectangle of cardboard measured 20x32. It should be approximately equal to the measurements of the poster frame less 4 in.
From another piece of corrugated cardboard, cut the following:
2x - 2.5 in. x (Length of the cardboard rectangle)
2x- 2.5 in. x (Width of the cardboard rectangle)
Retrieve the cardboard frame and duct tape the slats to the inside edges of the frame, perpendicular to the plane of the frame, making sure to tape them securely.
Step 4: Assemble the Light
Be sure to leave enough slack in the wiring for the switch to reach the edge of the rectangle.
Duct Tape the power cord to the bottom corner of the rectangle.
Step 5: Piece Together the Light Box Part II
Secure it with duct tape, and seal off all cracks where light can pass through.
Flip the whole thing over.
Line the switch up with the sidewall where you'd like it to be placed.
Cut a hole in the wall to accomodate the switch, and duct tape into place.
Step 6: Reassembly
Secure the cardboard with the fasteners on the frame, pinning the poster between the cardboard and the frame.
Hang the poster, plug it in, and enjoy!