Red shiso leaves are sold in bunches when it's time to make umeboshi (salted ume plums). I bought a 300 g bag.
Rip the leaves off the stems, since we'll only be using the leaves. Wash the leaves very well in water, and drain into a colander.
Bring 2 liters of water to a boil in a large pot, add the red shiso leaves and simmer to extract their essence. When the leaves have lost their red color and turned green, the water will have turned purple. Simmer for about 5 minutes, and then either squeeze out the leaves and remove them, or strain the liquid through a sieve into another pot.
Return the strained liquid to the pot, add the sugar and dissolve. Turn off the heat and add the citric acid (or vinegar). When you mix it the liquid will turn a beautiful red.
Leave to cool, and store in the refrigerator. If you made too much you can freeze it. Dilute to taste.
Citric acid is sold at pharmacies. 50 grams costs about 400 yen.
Notes: I've tried grain vinegar and apple cider vinegar as the vinegar. If you add a tiny bit of salt to the concentrate, you can drink it as a sports drink.