Slice the oranges and lemon into small pieces. I usually quarter them, then slice the quarters.
Place a bowl on the digital scale and press the "tare" or "zero" button. Add the orange and lemon slices to get a weight for the fruit.
Press the "tare" or "zero" button again. Add in half the weight of sugar. (If the fruit weighed 800 grams, add 400 grams of sugar.)
Mix the fruit and sugar with a spatula or large spoon. Once well mixed, transfer the fruit and sugar to a very large jar or pitcher. I use a spatula to get as much sugar as possible from the bowl into the jar.
Pour the soju on top and stir until well mixed.
Cover and let sit at room temperature for approximately a week. For the first few days, a layer of sugar will likely settle on the bottom, but will slowly dissolve. You can speed up this process by stirring it daily (or shaking it if it's in a jar with a secure lid).
Once the sugar has dissolved. Strain the soju and lightly press the fruit to extract more juice and alcohol.
Bottle, chill, and serve. See notes below for serving suggestions.