Grow a male and female plant of diffrent strains and let the male pollinate the female, then plant the seeds that come from the female, pick the best female and the best male, and repeat until its stable.
Someone corresct me if Im wrong, but I think you just cross breed two different strains that havnt been mixed yet.
The first generation of plants grown from seeds should be very similar (F1 hybreads), subsequent generations from of the same line can be highly differential though so to get a strain that will allways produce offspring of similar traits you have to keep inbreeding and selectiing hte same traits you want out of the final product.. this is kind of what happens naturaly but with a much larger pool of geanetics and not so much inbreeding. Trouble is it takes a lot of work and a LOT of time so very few strains are truely stable (durban is one stable strain I want to try to get some seed stock from for many future grows) and people mostly work with clones and F1 hybreads of clones they like. Most seeds sold are F1 hybreads of armys of two different clones so they basicaly all have the same parents and look/behave similarly. But dont take my word for it, I got a D in botany. Go to the libarary and read some books on plant genetics and breeding.