Get a male plant which is strain A, get a female plant which is strain B, rub the males pollen on the females clit, sorry i mean calyx. flower till completion. vuala, seeds are a new strain
If you're talking about stabilizing a phenotype to produce a consistent progeny, known by breeders as a "true breed," then there's lots of resources to help you understand the basics. Check out the breeding forum. But, if you cross 2 plants, the new plant is a new strain. It would correctly be called an "F1 Hybrid" of the two plants...such as a strain I'm growing called... AK-47 x Cherry Bomb F1 Hybrid ("AK Cherry Bomb") This strain is great, but if I were to seed it out, I wouldn't get the same strain from the seeds. As an F1 hybrid, the genotype is extremely unstable, and any breeding will result in a mixed bag of genotypes, and phenotypes as a result. However, if I seeded my NYC Diesel, a truebreed, the seeds (should) be similar to the parent, with maybe some minor mutations.