Fire up your laminating machine and laminate a sheet of paper for each layer in your design. Laminate a few extras too, just in case something goes wrong during the cutting.
I used regular printer paper with 5 mil laminating pouches. Feel free to play around with different types of paper, but be sure whatever you're using is a bit translucent - you should be able to see light through it if you hold it up to the window. This will ensure your layers all glow nicely.
You'll also want one completely opaque sheet for the frame layer in the very front, to block any light leakage from the NeoPixel strip along the sides. You can use laminated black paper for this if it's handy.
My black paper wasn't quite opaque enough, so I ended up using black sticky-back vinyl (the kind these cutting machines are designed for). I was able to stick the vinyl directly to the glass front of my frame so it won't move around and let any light leak out.
Cut all the other layers using a strong grip mat. You'll find more instructions and cutting tips on the Design page.
If needed, trim the corners off your layers with a pair of scissors. I found this really helped the layers to sit flat inside the frame.
Place the first white layer directly down onto the frame layer. Don't attach it to anything yet. We'll build up the other layers on top of this one and it'll be easier to adjust them all later if we keep things removable.
Stick a 3d glue dot down in each corner, making sure they don't show through the front of the design. These dots will function as both adhesive and spacers between the layers. I found that stretching and rolling the glue dots before sticking them down gave me the height and profile I wanted.
Stack the layers up one at a time, using the zots to space them apart. Your goal is to build up all the layers to be about the same height as the LED strip, with the final layer behind all the lights.
Test the placement as you go. I found that adding a few more spacers between the layers in the middle of the design really gave my shadow box the depth I was looking for.
When all the layers are piled up to your satisfaction, attach the MagTag to the back of the frame so it shows through the window in your design. Stick it on with a handful of zots or with magnets to hold it in place through the last layer.