If you're reading this, I assume you are interested in the theoretical transformation of a relatively weak alcoholic mixture into a relatively strong alcoholic mixture. That is, the distillation of whisky.
If you don't know about the early stages of whisky distillation, here is a quick round-up:
Take some grain, and allow it to sprout. Just as it starts to sprout, quickly kill it by drying. It is now a "malted grain". Mix the malted grain with hot water and stir until you get bored - you are dissolving the sugars from the grain into the water. Filter out the solids, and add yeast. Keep the mixture slightly warm (and sealed from the air) until the yeast has turned the sugar into alcohol. You now have a wash that is ready to be distilled. Apparently, the wash has a strength and taste similar to beer, so maybe you would like to start there.
Distillation is the process of separating a mixture of liquids with different boiling points. In this case, we're trying to separate ethanol (alcohol) from water. Pure ethanol boils at 78.4oC, and pure water boils at 100oC, so heating the wash will make the ethanol boil off first.
Step 1: What you need
I will refer to these parts as the vat, column and condenser. You also need a thermometer with a scale that goes to at least +100oC.
Legal point: It is illegal to manufacture spirits in the UK without a distiller's licence which is required under the provisions of section 12 of the Alcoholic Liquor Duties Act 1979 and this includes manufacture for "own/domestic use". For this reason, my images are a mixture of diagrams and stock photos. This goes against the usual practice here, but I kind of want to keep my job, and if I did it for real, images posted here can (in a UK court of law) be used as evidence against me. Before constructing your still, you must check local licensing laws to ensure you are not committing an offence, or obtain a distiller's license.
Since this is more a guide to function than form, you may choose to use different materials to those suggested, such as paying out for all-copper fittings. This is by no means an exhaustive tutorial, so if you are planning to produce quality drinking-spirits on a regular basis (as opposed to something merely flammable), you may even want to invest in a purpose-built still. Just remember (again) that, in the majority of countries where you can read this Instructable, you need to check the legality of distilling alcoholic beverages for personal use.