The questions are how many seeds do you want to make, and do you want to get any sensi buds from the same plants?
If you want to seed the entire room to maximize the amount of seeds you get, wait until about 28 days after flip, when there’s max pistils but none are dying yet, turn off the exhaust fan, then take a dab tool, dip it into the pollen, and put it in front of your circulation fan inside the space. Let it sit and circulate for an hour before you turn the exhaust fan back on. Repeat the same process in two or three days, then wait a week to see if the pistils die back to show you’ve gotten good pollination. Let the plants grow for about 2-3wks longer than you normally would to ensure maximum ripening.
If you want sensi buds on the same plants and only want to make a few seeds, it’s quite a different approach. Don’t need a brush - just use a qtip for ease and disposability. There’s a lot of techniques, but I find turning off exhaust fans, pulling your plants out into a well ventilated area like a bathroom with an exhaust fan going, helps prevent pollinating other stuff. Dip the qtip into the pollen, roll it on the buds you want to pollinate, being careful not to spread it around. Let it sit for 4hrs or so and then spray with water to deactivate any remaining pollen, put the plant back in your space, and call it good. Or you can put a paper bag over the bud/branch and twist tie it off to leave the pollen on for a 24-48hr period if you want with the plant back in the grow space, but that’s totally unnecessary. When you take that paper bag off off, do it outside the grow and spray the branch and bag before, during, and after with water to deactivate any rogue pollen.
Hope that helps!