<"I'm the unknown stuntman that makes Eastwood look so fine..." (know what tv series this quote was from?>
Cross breeding is easy.
1. Put the male plant in a different room (or any way you can isolate it from the female).
2. Keep the 12/12 going.
3. Don't use a fan.
4. Wait until the sacs release pollen all over the leaves. It'll look like it snowed on the plants.
5. Use a little baggy (sandwhich or ziplock, etc) and collect the pollen in the bag.
6. If the female plant is the ONLY female plant in the grow area then bring the bag of pollen to the plant. If the female is among other plants, move the female plant to a separate area away from the others.
7. Get a rubber band (or twisty tie or whatever). This will be used to mark the branch you've pollinated.
8. Find a branch you want to pollinate.
9. Open the bag of pollen just enough to stick the end of the branch into the bag.
10. Seal the zip lock around the branch as much as you can.
11. Wrap a plastic grocery bag over the zip lock bag (this is like double security in case some of the pollen escapes the zip lock bag).
<At this point, you basically have a ziplock of pollen with a female branch in the pollen bag, which is in side a grocery bag.>
12. Shake the branch inside the ziplock vigorously.
13. Through the grocery bag, take off the ziplock being careful not to let any of the pollen escape (if some does, no worries, that's why there is a grocery bag).
14. Now that the zip lock has been removed, seal the zip lock through the grocery bag. Shake the newly pollinated branch to get any loose pollen off.
15. Remove the branch from the grocery bag.
16. Put the rubber band on the branch so you'll know which branch has been pollinated.
17. Wait until harvest. You'll have an entire but FULL of seeds.
...and there you have it. An isolated branch which has been pollinated.
So...I can hear the question now....
"Which branch should I pollinate?"
It's up to you. Most will NOT pollinate the top cola. If you do, you'll get a shitload of seeds (because the bud is the biggest) but you'll also get less to smoke. If you choose the bottom branches you may not get fully developed seeds (depending on how optimal your conditions are) and not as many seeds.
There is no right or wrong answer. Maybe somewhere in the middle might be best.
Coincedentally, a friend just harvested this last weekend. We counted 60 fully grown seeds from a branch that was three nodes down.
Let me know if you have any specific questions.