Soju is, in its original definition, is a distilled spirit produced from fermented rice, the most popular grain in the region of the origins of soju, the Korean peninsula. Up until the 20th century many homes distilled their own soju, with both regional and personal characteristics. It was considered to be part of a typically Korean culture that Korean natives could identify with; it was therefore subject to prohibition by the Japanese, who had colonized (or “merged” as was the term applied) Korea from 1910 for thirty five years until the end of World War II.
The traditional form of soju is still very rare and hand-crafted and sold only in certain parts of the Korean peninsula; it contains over 45% alcohol by volume. It is rather a shame that the traditional soju is no longer very popular or known well, when the contemporary, diluted form of soju is known by many around the world.
What is known as the present form of soju is actually a diluted form, with only 25% or less alcohol by volume. A sweet flavor is a characteristic of the contemporary soju, because it is actually highly distilled ethanol with sweet potatoes, diluted with water and flavored with sweeteners. The alcohol by volume is encouraged to be decreased, and has indeed been decreased, partly due to the immense intake of soju by the Korean public.
Soju is now exported to around eighty countries, being a distilled spirit of sorts, albeit much weaker than the typical whiskey, vodka or even schnapps. Many Koreans still call it a distilled spirit, but it is technically a diluted and sweetened alcoholic beverage. Because of its low alcohol by volume, it does not require a special permit as strong liquor or spirits do in the United States; that and the large number of Korean Americans have led to the selling of soju and soju-applied cocktails alongside beer and wine at regular western restaurants.
To make the real soju as it was meant to be, however, should be quite easy when you have a proper still and some rice. It is rather like making whiskey but using only rice. Rice has a slightly sweet flavor that barley or rye do not, and a mild after-taste that wheat does not have, and which is imitated by the diluted form of soju; unlike the diluted soju, proper soju can be an interesting alternative to the regular whiskey or rum. When properly distilled, soju can be said to be the Korean equivalent of moonshine. It takes a little more hand work than the usual fruit mixes but not difficult. You would need, as with all other home distilled beverages, a Moonshine still, devices to heat your mash of rice out or doors, and a good amount of rice and yeast. The traditional way of making yeast is to hand-craft it by kneading some dough of wheat and water. The rice is to be steamed then stored in a jar together with the yeast for a fortnight. Distill at once but over a long time until the flavor seems right. Cooking the rice in a steamer and using ready-made yeast will be the much easier alternative.