I have a lot of 0% beer at home and I wanna make them 8%-18% alcohol. I've been said that if I add yeast and sugar to 0% beer, it will become alcoholic beer. Now I need proportion of yeast and sugar for 1 Liter beer and the process to make it happen?
All Ethanol is "natural" in the sense that it comes from fermentation. Neutral spirits are created through distillation to concentrate the alcohol. In theory you can combine sugar, water, yeast to make something that is kind of neutral in flavor, but that will only get you to about 16-20% alcohol. So you would have to dilute the 0% beer with this 16% brew and therefore dilute the flavors of the original beer. Far more effective, would be to buy some vodka and dump that into your beer (Unless you are in a country where you can't buy it, which is what I suspect). Look up "Sugar Wine" recipes like this Sugar Wine recipe.