As you all probably know, cannabis plants can be female or male (or intersexual but those hermies will ruin your crop)

In order to produce feminized seeds you need to spray a female plant with certain chemicals, usually with something called STS (Silver Thiosulfate) just before they turn to flowering stage. That inhibites the ethylene production, and the female cannabis plant will grow male flowers, which will release pollen.

Apply that pollen over a fertile normal untreated female plant, and she will develop feminized seeds.