So, here's the one I did:
What you need:
Black paint (or Black Sharpie permanent markers)
Glue (I use Elmer's Craft Bond Extra Strength glue sticks)
Your computer's printer, or....
Possibly scrapbook paper, if you'd rather use something with just a pattern on it than actual images
Embellishments of your choice: metallic Dresden trim, skull beads, ribbon, etc....
The decorating part is very simple:
Paint the coffin black
Wait for it to dry
Glue some stuff on it
That's really all there is to it. I glued some silver Dresden on the cover, and glued some print-outs of coffin innards into the inside and that was it. The only real tip I have for this one is that it does make things a lot easier if you take off all the hinges and the clasp and completely separate the two halves of the coffin while you're doing the painting part.
I made my own coffin innards in Photoshop, and that ended up being the most involved part of the whole thing. It was easy to find an image of some ivory satin that I could digitally turn red, but finding the pillow was way harder. After spending an hour in Google Images trying to find a picture of a pillow that I thought would look right in a coffin, it finally dawned on me to search for "ring bearer pillows" and that was finally that. In Photoshop, I used "Hue/Saturation" and "Selective Color" to age the look of the pillow to something less bright-white and fresh-looking.
After the aggravating amount of time it took to find the pillow image, I thought I was in the clear, but finding an image of a skeleton I could use turned out to be impossible. There are tons of images of skeletons to be found online, but every one of them was, like....posing, I guess. Not a one of them looked like it was laying flat out. So I ended up taking a photo of one of my little plastic skeletons and using that. It's suddenly occurring to me as I type this that I could have detached its forearms in Photoshop and crossed them over its chest. Oh, well.
So, there it is. Have fun!