It’s a simple method: brew hot (using a pourover brewer like this Hario Dripper or Melitta single cup coffee brewer) and cool instantly by substituting half of the water you’d normally use for making hot coffee with an equal amount of ice cubes. As a rule of thumb, once ice cube equals 30g.
On his personal blog, Giuliano details the scientific reasons why this is a superior method to cold-brewing (namely: coffee’s solubility, volatility and oxidation). The Japanese Iced Coffee method helps extract the most flavour and prevent off-flavours, because you’re brewing hot first. It certainly is a quicker method of getting delicious iced coffee — no need to wait 12 hours or more.