Take a plant at preferably a small height(1/2-1 ft) and top it.Wait a couple weeks for 2 nice shoots to develop and then take a cutting about an inch before the shoots.Root it with your method of choice.Stuff it into your medium of choice after its rooted, about 1/2 inch past the bottom of the shoots.LST the 2 tops to get them as far apart as possible.When comes time to pollinate your stuff take bags of appropriate size and fill it with pollen by shaking male plants with the bag around it or just adding stored pollen, put the bag around the shoot. Go to the other shoot and pollinate it using the same method and pollen of your choice.Remove the bags after its Pollinated you will have 2 different f1's/crossed seeds within 1 plant .

fixed a word for ya.

That's a lot of work.

I've done 5 different crosses on one plant by putting a garbage bag over a big plant but leaving one lower branch exposed and pollinating it with one strain. Next day spray that branch with water to deactivate any left over pollen then repeat above 'til I run out of different pollens.

Different coloured twist ties and notes on what's what and in 6 weeks or so the seeds are ready.

The rest of the plant can be harvested sooner just leaving the pollinated branches intact until the seeds are ripe.

Easy-peasy!