>>152169381. Buy clear 3DS cover
2. Print out picture, cut to size
3. Put picture under clear case how you want it, trace out the holes, edges of case, etc.
4. Cut out accordingly, put it back in and you're done!
Some trial and error required as far as print size, and figuring out how rounded the corners should be. I had a sort of template, but by the time I modified it (it was a template for sticky decals, so it was shaped so the material would lay flat on corners, which wasn't necessary, and it was too small), it was basically just a rectangle with a couple rounded edges.
I can't entirely recommend the case I got either (
http://www.play-asia.com/barikata-cover-3ds-ll-clear-paOS-13-49-en-70-5717.html ). It has good, even pressure so you can be sure the paper won't move, but the location of the fasteners means the top left/right (just under shoulder buttons) aren't completely immobile, so they can creak/squeak a bit when you push on them. It's rather minor honestly, but something to be mentioned.
>Previous inserts in which I followed the template without any modification other than sizing it up 