On an outside grow the flowering stage begins around the Autumnal Equinox, inside it begins when you switch to a 12 hour on 12 hour off light cycle. The first 10 days or so are a pre-flowering period where the plant switches from producing lots of leaves to producing flowers. During this pre-flower it will become evident if your plants are male or female, remove all males if you don’t want seeds (there will be tons of them).
You’ll want to keep your “dawn” time the same when you adjust your light cycle, e.g. if your vegetative light cycle ran 6am to midnight, while flowering your lights should run 6am to 6pm. The lighting itself should change to 2700k (instead of 6300k for vegetation), which has a reddish hue and mimics the fall sun.
You want to maintain a temperature between 75 and 80 degrees during the lights on period of your light cycle, with temperature dropping into the mid 60s while lights are off.
WATERING AND NUTRIENTS
You should keep your watering regiment the same as you did during the vegetative state, watering when the plant’s top inch or two of soil has become dry. However a switch in nutrients from a high nitrogen fertilizer to a high phosphorous fertilizer with potassium, magnesium and some sulfur (along with other micro nutrients) will be very beneficial to your plants, and your overall yield.
Maintain a humidity level between 50% and 60 %. Dropping below 50% can cause issues that resemble nutrient deficiencies, light/heat stress. High humidity can result in mold or other issues.
It is important to maintain the circulation you established during vegetation, never blowing directly on your plants, but allowing a continuous air exchange providing fresh CO2 to your plants.