Today, St. Patrick's Day, I passed these out to my friends. Next year I'll have to make more because even though i made nearly three dozen, I ran out in less than fifteen minutes.
Step 1: Supplies
-Plastic, milk jug sides work great
-Vacuum Former. Sheekgeek has a cheap and easy was to make one How to Make Your Own Prototypes: How to Make Your Own Plastic Vacuum Former
-Green food coloring
-Spoon(s) and Knives
Step 2: Make Chocolate Mold
Again, check sheekgeek's instructable for great directions on how to form.
When I made my shamrocks, I doodled a decent shamrock on some scrap wood and went at it with the band saw. Don't cut yourself.
When heated enough to form, milk jug sides will turn semi-transparent. For a good form make sure the entire piece of plastic is heated until it's nearly sagging. The quicker you get the hot plastic on the former and the vacuum on, the better it will come out.
Step 3: Melt Chocolate
Make sure you don't run out of water, it smells horrible and can ruin the pot.
Step 4: Mold Chocolate
The chocolates will probably have some extra that you don't want. Tidy the chocolates up with a sharp(er than a butter knife) knife.