In a medium saucepan, whisk together the juice and agar powder. Let sit for 5 minutes. Meanwhile, stir together the cornstarch and water. Set aside.
Bring the juice mixture to a boil. Once boiling, let simmer for 1-2 minutes. Then turn off the heat and add the cornstarch mixture and alcohol. This is also when you will add the blackcurrant or lime juice, depending on which flavor you are making. Stir together and let sit for 3-5 minutes.
Prep your syringes. Pull apart the tops and make sure the caps on the “needle” portion is securely fastened, then lay them out on a surface like a flat baking tray. Put your juice mixture in a pitcher for optimal pouring. Fill and cap the syringes one at a time, then prop them in an upright position while they chill. Let chill in the fridge overnight or in the freezer for 30-40 minutes, then enjoy!