Hint: Don't try this on yourself before a doctor's appointment, it took me a while to explain them, that everything was fine with me despite my slightly green tongue (usually it wears of pretty quickly, but I was testing different mixtures) and a rash (I'm allergic to chocolate...).
Step 1: Ingredients
• Chocolate (white or dark, though dark works better)
• Green powdered food colouring (I tried Kool-Aid "Lemon-Lime", but the amount you need to colour somebodies tongue adds a very strong lemon taste to the chocolate. It's definitely better to use powder without taste)
Other stuff you need
• A shamrock chocolate mold (e.g. here)
• A small and a big pot
• Sharp knife and cutting board