If we had to describe gin, perhaps the simplest definition would be vodka incognito. Essentially, gin is nothing more than a botanical-flavored neutral spirit and neutral spirits, as we are all aware, are more commonly known as vodka. Your basic gin, then, is created from a distilled blend of vodka with botanicals. The main botanical used is called the juniper berry, a blue-ish green berrylike fruit. However, creativity and diversity arrive when a slew of herbs and spices are thrown into the mix. This is also where master distillers conjure up their secret recipes.
First, your neutral spirit should be diluted down to about 35% alcohol by volume. (Note: when spirits are cut to below 45% ABV, a slight turbidity will occur. To reattain distillate clarity you will need to filter your product.) Once you have loaded the base spirit into your still, you have two choices: to suspend your botanicals in a tray above the kettle or to soak the botanicals directly in the spirit. Traditionally, trays were placed above the kettle (just at the base of the helmet) so that as the vapors rose from the heated ethanol they would strip all the goodies from the herbs, such as flavor and aroma. This would pass through the condenser and what resulted was a fine-flavored gin. The herbs in the tray can be dried or fresh. Naturally, the dried herbs impart less character than fresh berries and a larger quantity is thus needed to achieve a certain degree of flavor. On the other hand, while fresh berries will yield a stronger bouquet and less is needed, the cost of the raw material is higher.
Nevertheless, the current and more efficient method of distilling a gin would be to soak the botanicals in a high-proof spirit (again, diluted down to about 35% ABV) for anywhere between 8-12 hours while slowly agitating the mixture and then redistilling the blend. Depending on the quality of your berries, you will need approximately 15 kilograms of berries for every 100 liters of high-proof spirit. If suspending your berries in a tray, you can estimate you will need up to three times the quantity to achieve similar results.
As mentioned earlier, not only juniper berries are used to create gin. If everyone used the same ingredients there would be no distinction, no competition and no choice! So, the beauty lies in the artistic prowess of the distiller. Other herbs typically used in making gin include anise, coriander seeds, orange or lemon peels, nutmeg, fennel, cardamom, cinnamon, etc. You can guess that some of these herbs will result in a product not too far from ouzo, for example.
Finally, you probably want to know what kind of still you need to make gin. The typical configuration is quite simple: a kettle with a top-mounted helmet and no column. This style has been used historically because reflux columns were not yet widely used. They are still acceptable today, as well, because your base spirit is already clean, neutral and high proof. However, you will find that some distillers do use reflux column stills for creating gin. For one, they can make their own high proof. Two, and importantly, you can further clean the vapors that have soaked in or passed through the botanicals. Off-flavors and undesirable elements strip off the herbs just as the good and desired elements do. Through a reflux column, however, the new spirit has a chance to be freshly cleansed.
However you decide to make your gin, be creative and artistic, and the gin connoisseurs will seek you out!