Starting the flowering period
Since cannabis regular checks when the daylight times are changing and your not living in India, than you have to simulate the natural habitat a bit. If you are using auto flowering Cannabis strains, than you don´t have to care about this part. All others have to switch the light to a 12-hour period per day. This will mimic the change in daylight from the natural habitats.
But it is also essential to count in other aspects than only the times of light per day. The flowering period also requires another spectrum of light, for Indoor growers this regular means they have to use a sodium discharge lamp or an equivalent lamp.
Another important point is to switch the schematic of fertilizing, if you are fertilizing at all. The Cannabis plants now require more potassium and more phosphor than during the cannabis growing period. Therefore you can find a large assortment of ready to use fertilizers in your headshop, for all growing stages of your cannabis plant.
During the first flowering weeks
Within the first week the changes are only minimal and you won´t be able to see a real big change on your Cannabis plants. Within the second week that changes and you will now be able to see small white hairs or small green knobs.
If you haven´t been using feminized Cannabis seeds, it is now your last chance to take away the male plants from the females or you will end up with buds full of seeds and very low content of THC.
All plants with white hairs are female Cannabis plants and can luckily stay where they are, all Cannabis plants with green knobs are male plants and will build pollen within the knobs.
Most growers simply destroy the male Cannabis plants, I know it hurts to crush a plant you put in much love and work, but trust me, they won´t be useful to you in any way.
Now that there are only the female plants left, you have to ensure that all of the flowers get full light to receive best results. Another important thing is to turn the plants every 2 days a bit, this is to strengthen their stems and to avoid that they snap under the weight of the flowers. This is especially a threat when you use Cannabis strains with enormous large buds. In some cases you also have to tie the stems, to that they can´t bend too much or even make the plant snap.
Later stages of the flowering period
As said, it depends on the strain you are using how long the single stages of the flowering period are taking. But to give an average, you will see a new change after 5 to 6 weeks. The first white hairs of your Cannabis plants will start to become brown. That doesn´t mean your plants are ready to harvest, it is just a sign for maturing.
In this stage of the flowering period the plants are very fragile to bud mold. That means you have to check the humidity carefully and it shouldn´t go over 40%.
If you are now going into the last stage of flowering, than you also should stop fertilizing 10 to 14 days before the harvest.
Otherwise it is possible that you still got a residue from the fertilizers within the Cannabis flowers. In most cases they won´t be hazardous to your health, but they could adulterate the taste in a very bad way. You also wouldn´t smoke a cigarette that has the smell of dog poo, won´t ya?
When you stopped fertilizing, you should spray your plants with plenty of water every day. So that your plants can use up the leftover nutritions and get rid of the leftover fertilizers.
The last 2 days of flowering
You can stop watering your Cannabis plants now, this speeds up the later drying process and will not lower the quality of the taste, smell or content of active components.
I also just came across another way to give your plants the final touch, turn out the lights and keep them in the dark during the last 2 days before harvesting. The lack of light will make your Cannabis plants produce high amounts of resin, as the maturing process speeds up. I checked the web for this technique and most growers reported nice results with this technique.
To check it yourself, you can keep some plants in the light and some others in the dark to verify it yourself.
Oh and sure, stop watering and switching of lights can be combined.