This process requires a knowlege of the genes which are present and their effects, and the technology to make the desired changes.
Recombination occurs naturally.
We can build a DNA library by putting pieces of DNA from an organism into plasmids. Imagine that each bottle of identical plasmids carrying a particular gene is like a page in a book. If we had enough time and money, we could make a library with rows and rows of bottles, each containing plasmids dissolved in water, so that all the genes of our entire genome are stored and can be retrieved, one at a time, if needed. Restriction enzymes cut the double stranded DNA at specific sites. DNA probes are complimentary sections of DNA that can be used to locate a gene.