According to the BBC, the team of medical professionals, headed by Valery Zukin, the director of a Ukrainian clinic of reproductive medicine, used a method called "pronuclear transfer" to create the viable embryo from which the infant was born, fertilizing the mother's egg with the father's sperm and then transferring those combined genes into a donor egg.
Technically, this baby is not the first 3-parent baby – another baby was born in Mexico last year, using a different method of in vitro fertilization.
The 3-person IVF procedure was originally developed to prevent mothers with unhealthy mitochondria from passing along genetic disorder to their babies, by subbing in the healthy mitochondria from the donor female.
She did not have mitochondrial disease, so this is considered the first 3-parent baby where faulty mitochondria wasn't an issue.
Some in the field believe that the procedure should have been restricted to male babies, since (biologically) males with donor mitochondria can't pass their own modified genetics on to their offspring (see also: Human Reproduction 101).