The following passage is specifically for Havana cigars, but the process is, broadly speaking, similar elsewhere.
Cigars are a natural product, often compared to wine (though the comparison sometimes tends to get out of hand). The quality of a cigar is directly related to the type and quality of leaves used in its construction, just as the quality of wine depends on the type and quality of grapes used.
Tobacco seedbeds have to be in flat fields, so that the seeds aren't washed away. After being planted, the seedlings are covered with cloth or straw to shade them from the sun. This covering is gradually removed as they begin to germinate, and after around 35 days (during which the seedling will be sprayed with pesticides), they are transplanted, usually in the second half of October, into the tobacco fields proper. The leaves are watered both by rain and the morning dew, and irrigated from below.
The tobacco plant is considered in three parts: the top (or corona), the middle, and the bottom. As the leaves develop, buds appear. These have to be removed by hand to prevent them from stunting leaf and plant growth. The quality of wrapper leaf is crucial in any cigar. Plants called Corojos, specifically designated to provide wrapper leaves for the very best cigars, are always grown under gauze sheets held up by tall wooden poles. They prevent the leaves from becoming too thick in a protective response to sunlight. The technique, called tapado (covering), also helps them to remain smooth.
When harvesting time arrives, leaves are removed by hand using a single movement. Those selected as wrappers are put in bundles of five, a manojo, or hand. The leaves are picked in six phases: libra de pie (at the base), uno y medio (one-and-a-half), centro ligero (light center), centro fino (thin center), centro gordo (thick center), and corona (crown). The libra de pie section isn't used for wrappers. A week passes between each phase. The finest leaves found in the middle of the plant; the top leaves (corona) are usually too oily to be used for wrappers, except for domestic consumption, and are often used as binder leaves. The whole cycle, from transplanted seedlings to the end of harvesting takes some 120 days, with each plant being visited an average of 170 times making it a very labor-intensive process.
Wrapper leaves grown under cover are classified by color as ligero (light), seco (dry), viso (glossy), amarillo (yellow), medio tiempo (half texture), and quebrado (broken), while those grown under the sun are divided into volado, seco, ligero, and medio tiempo. The ligero leaves from the top of the plant have a very strong flavor, the seco from the middle are much lighter, and the volado leaves from the bottom are used to add bulk and for their burning qualities. The art of making a good cigar is to blend these, along with a suitable wrapper leaf, in such proportions as to give the eventual cigar a mild, medium, or full flavor, and to ensure that it burns well. The leaves are also classified by size (large, average, small) and by physical condition (unhealthy or broken leaves are used for cigarettes or machine-made cigars). If all the leaves are good, each wrapper plant can wrap 32 cigars. The condition and quality of the wrapper leaf is crucial to the attractive appearance of a cigar, as well as its aroma.
The bundles of leaves are then taken to a tobacco barn on the vega, or plantation, to be cured. The barns face west so that the sun heats one end in the morning, and the other in the late after-noon. The temperature and humidity in the barns is carefully controlled, if necessary by opening and closing the doors at both ends (usually kept shut) to take account of changes of temperature or rainfall.
Once the leaves reach the barn, they are strung up on poles, or cujes, using needle and thread. The poles, each holding around 100 leaves, are hoisted up horizontally (their position high in the barn allows air to circulate), and the leaves left to dry for between 45 and 60 days, depending on the weather. During this time, the green chlorophyll in the leaves turns to brown carotene, giving them their characteristic color. The poles are then taken down, the threads cut, and the leaves stacked into bundles according to type. The bundles are then taken to the fermentation houses and placed in piles about three feet high, covered with jute. Enough moisture remains in the leaves to spark the first fermentation, a process like composting. Heat develops, but the temperature must be watched carefully so that it does not exceed 92 degrees F during the 35 to 40 days that the piles are left intact. The leaves assume a uniform color.
The piles are then broken up and the leaves cooled. The next stop in their journey is at the escogida, or sorting house, where they will be graded according to color, size, and texture and where the fillers will have part of their stems stripped out. In preparation for handling, they are moistened either under a spray of pure water for wrappers or a mixture of water and the juices from tobacco stems for fillers.
Traditionally, women perform the tasks of sorting and stripping. Each leaf is tenderly examined and graded. Broken leaves are set aside for use in cigarettes.
Flattened onto boards (planchas), the leaves return to the fermentation area. In dark rooms, they are built into stacks called burros up to 6 feet high. The second, more powerful fermentation begins within the damp leaves. A perforated wooden casing has been buried in the burro, into which a sword-like thermometer is thrust. The temperature inside must not exceed 110 degrees F for around 60 days (longer for some leaf types, shorter for others). If it does, the bulk is broken down and the leaves cooled before it is rebuilt. Ammonia is released as the leaves shed their impurities.
Because of the fermentation process, cigar tobacco is much lower in acidity, tar, and nicotine than cigarette tobacco, making it much more palatable.
It is now time for the leaves to be sent to the factories or warehouses in tercios, square bales wrapped with palm bark, which helps to keep the tobacco at a constant humidity, and slowly mature until it is needed, sometimes for as long as two years.
These long and complicated processes of selection and fermentation have to be carefully supervised and are crucial to the final flavor of handmade cigars.