Like with most things smokable, growing is easy, harvest and curing is not. I will try to give a brief explanation on the process.
Most commercial operations buy their plants at approx 2 inches tall from a local supply source. If starting from seed it should be started inside by October - November.
After the ground thaws and there is no danger of frost (March - April) the plants are sown in rows with approx 36 inches between plants. These will grow until late August - late September (depending on how far north the crop is), during this growth cycle they will be topped between 2 and 4 times. Topping is cutting off the flower so no energy is used to produce seed, this makes larger better tasting leafs.
Harvest time comes when the leaf mass turns from green to a even yellow color, looks like the plant is starting to die. Then the plants are cut approx 3 inches above the ground and staked. Staking is where three to five cut plants are place on a stick by puncturing the stalk approx 3 inches above where it was cut. The staked plants are left in the field for approx 14 days (so long as there is not much rain) before collection and hanging. When collected the staked plants should be hung in a tobacco barn (barn with 2 inch gaps between the outer boards), they should be hung so the stake is horizontal and the plants hang vertical.
The plants then hang until January or February or until the moisture content in the stalk is at about 10%. Now comes time for stripping the leaves and separation. They are normally separated into 3 types of quality, trash; b grade and a grade. This is easy, bottom leaves are trash, middle leaves are b grade and the upper pristine leaves are a grade. The three separate grades are then bundled and taken to a tobacco market where cigarette and cigar companies bid / buy on the bulk harvest. They then take the harvest and further cure the tobacco for 3 to 5 years in wooden barrels. The last curing process and time will vary depending on the use, cigar or cigarette and the companies recipe.
This entire process is for temperate (American) climates, it is a entirely different process in the tropics as there is no cold weather.
Hope this helps a bit,
nobody