Mix into a pot 2 cups of water, 3 1/2 cups of white sugar, 1 cup of light (important) corn syrup and 1/4 teaspoon of cream tartar
Place stove on medium-high and place a candy thermometer on the side of the pot to measure the temperature. Bring the mix to a boil (this takes a LONG time - the most time consuming part. I waited a good 15 minutes before I saw any bubbles at all) and let boil until the candy thermometer is at 300 degrees which is also considered hard ball. All candy thermometers are different and I swore I was doing this wrong at first, but don't worry because it does take a while to get to 300 degrees! Stir the mixture constantly before, during and as it boils to prevent it from hardening. As the water in the mix begins to evaporate, it'll thicken more and more (this is a good thing, but don't let it get too thick by mixing non stop!).
Have a baking sheet lined with either wax paper or tin foil ready near your stove. Once the mix reaches hard ball/300 degrees, quickly pour the mix onto the baking sheet. Place it in the fridge to cool completely.
If you're in a bit of a time crunch, you can try what I did the second time around; after letting the mixture cool for at least an hour, try placing the baking sheet(s) into the freezer for 5-10 minutes at a time and then back into the fridge. When I originally made this, I made the sugar glass sort of in a rush and I only let the mixture cool in the fridge for 2 hours... When I took the mix out, it was still sort of gooey and not completely hardened. Having the mix not 100% hardened will make it super hard to break up (and will melt quickly) once you get to that step!
Just by feeling the "glass", you'll be able to tell if its hard enough or not. If you have the time, place the baking sheet into the freezer for the last 20 or so minutes before you're going to use it to help harden it as much as possible.
Take a rubber mallet and smash the glass into shards. The more you smash, the smaller the pieces will get. I wanted some small and some bigger pieces, so I smashed it and then cut some pieces smaller with a razor blade as needed.
In a large bowl, mix together 1 tablespoon of cornstarch, 1/2 cup of light (again, important) corn syrup, 1/4 cup of water (add this slowly - sometimes 1/4 cup is too much and sometimes it isn't enough so eye the amount), red food coloring, blue food coloring, chocolate syrup (optional - this is just if you need the mixture to be thicker. There's other options, but who wouldn't want their edible blood to taste like chocolate?!)
First add red food coloring (approx. 12 drops) and blue drops as needed. I found that 12 drops of red, 3 drops of blue, mix it, and then 2-4 more drops of red works best. Mix it until it thickens and add the chocolate syrup if necessarily/wanted. You may not need it, but a little bit (not too much or it'll turn brown!) does help the thickness.