Can anyone tell me how to force flower out door plants?

cant do it.
what i did beacuse i like the flowering the outdoors bring, i started seedlins in a indoor setup, until the reach sertan level, then its aclimatization time, this is done very slowly, then in a week the plant would be outdoors when 12/12 is on abiously.

Force flowering depends on the strain, the technique, and the the length of day at your latitude.

One technique is to start flowering indoors and place them outdoors at the beginning of July. Some plants can flower indoors at 14/10 or 13/11. These plants won't revert back to vegging once placed outside. When you take a indoor plant and place it out doors, it will react to the sun powerful rays. Sometimes this reaction is harmful to the plant especially if it's young. So it's a good idea to acclimate the plant for a week or so. Force flowering, this way, is more successful when place outside after the summer soltice(june 21).
Some strains may be differcult to force flower if you live in an area where the length of day is sixteen hours in July.

The other technique is to cover the plant, two hours before sunset or two hours before sunrise for two weeks. Some green houses in europe have automatic shutters that do this.
You can cover your plants with black plactic bag but there are several problems that can occur.
A black plastic bag can over heat the plant.
If you miss a day you might have to start all over again.
If you plant is wet when you cover it; it is a good way to give you plant mold.

Some sativa strains are differcult to force flower.