Kilju can be produced by fermenting sugar, yeast, and water, but kilju made exclusively from sugar, yeast, and water was illegal in Finland before March 2018;[2] therefore, grain, potatoes, fruits[3] or berries were used during fermentation to avoid legal problems and to flavor the drink. Oranges and lemons are a popular choice for this purpose.

The process is similar to that of homebrewing wine; it requires rigorous hygiene and filtering of the product. Properly made kilju is a clear, colorless liquid with no discernible taste other than that of ethanol. It can be refined into pontikka (Finnish moonshine) by means of distillation. It then resembles rum, as both are distilled from fermented cane sugar products, although rum is often made from molasses, a byproduct of the sugar refining process, or fresh sugar cane juice instead of refined, crystallized table sugar.

Kilju is often produced improperly by home brewers who allow contaminants to disrupt fermentation or do not adequately filter or rack the liquid, or do not use a fining agent. The latter mistakes result in yeast being suspended, causing the mixture to be cloudy rather than clear. The yeast is not harmful, but can provide an unpleasant taste.[citation needed]

When homebrewing grew in popularity during the economic depression that followed the Finnish banking crisis of the early 1990s, yeast strains known as "turbo yeast" were introduced to the market. These yeast strains enable a very rapid fermentation to full strength, in some cases in as little as three days (compared to several weeks required by traditional wine yeast strains). Such a short production time naturally does not allow the yeast to become lees. The introduction of turbo yeast reinforced the public's view of kilju as an easy method of procuring cheap alcohol.