Spray the bud sites of your known female plant daily during first 3-4 weeks of the flowering stage (until pollen sacs form and start splitting open) – After switching to a 12/12 light schedule, choose bud sites on your known female plant, and spray/drench them daily with colloidal silver (or gibberellic acid). As the treated flowers develop, they will form into male pollen sacs. Untreated bud sites on the plant will form into female buds as usual; however these buds are unsafe to smoke unless you've been very careful to make sure they didn't come into contact with colloidal silver or gibberellic acid during the feminization process.
Harvest "Feminized" Pollen – When pollen sacs are ready to be harvested they swell like a balloon and start to open up. Make sure not to harvest early and also make sure to keep spraying the bud sites daily until this point or you might end up with empty pollen sacs! When the pollen sacs are ready, the leaf section protecting the pollen will start to crack. At this point it's time to collect the feminized pollen. One of the easiest ways to do this is to collect the pollen sacs directly and let them dry for a week. At that point they can be placed in a bag and shaken to easily collect all the pollen.
Pollinate Another Female Plant – At this point, take the feminized pollen you've collected and use it to pollinate a female plant that has been flowering for about 2-3 weeks (full detailed instructions with a video on how to do this below). Although it's possible to pollinate the same plant as the original, it's not recommended in part because the timing doesn't match up (attempting to pollinate buds too late in the flowering stage leads to reduced seed production). It's best to pollinate a different female plant that you started budding a few weeks after the original. This increases the number of seeds produced as well as gives the new female plant enough time to develop them to maturity. It also increases genetic diversity compared to self-pollination.
Wait ~6 Weeks to Harvest Seeds – After about 6 weeks from pollination, the calyxes on the buds of your female plant will be swollen and fat. You know it's time to harvest your seeds when they start bursting out. At this point, it's time to congratulate yourself because you've got feminized seeds