Spread out the plastic Jello shot cups you'll be using on a baking tray (and make sure there's a spot large enough for the tray in your fridge). Set the tray to the side.
To a large heat proof bowl, add the Jell-O powder. Pour the cup of boiling water into the bowl, and whisk until the powder has dissolved.
Add the 1/2 cup of cold water, and whisk to incorporate.
Add the 1/2 cup of vodka, and whisk to incorporate.
Either pour the mixture into a liquid measuring cup (with a spout and handle) or use a turkey baster to evenly transfer the mixture to the plastic cups.
Refrigerate for 1 1/2 hours to 2 1/2 hours, or until the Jello shots have set. Store them in the refrigerator until you're ready to serve them.