Assuming you've played the game, you'll recall that there are several key cards you find throughout the game which progress the storyline. There are no spoilers here; you get the key cards regardless. If you're thinking about cosplaying someone from 999, these key cards might be a fun little addition to your outfit and/or group. Also, they're easy to make.
Step 1: What you need
-Old plastic card, like an empty gift card or hotel key card. You probably don't want to use a credit card, but it would be cool if you could get away with it.
-Electrical or other black tape
-Colored duct tape
Step 2: Preparing the card
Step 3: Laying down the color
Purple - Saturn
Blue - Mercury
Green - Uranus
Red - part of one of the puzzles (but I won't say which)
Yellow - one you find towards the very end of the game that I won't give out any more details on.