As Mikkel mentioned, hybridizing Citrus isn't necessarily as straightforward as most plants. Citrus can produce:
1. Nucellar embryos which develop into exact clones of the mother tree.
2. Zygotic embryos, containing genetic contributions from the seed parent, as well as the pollen donor parent, same as are normally found in the majority of plants.
3. Or commonly, a mixture of both types.
Citrus having exclusively nucellar embryos can serve as pollen parents. Citrus producing zygotic embryos can serve as either pollen, or seed parents. There are additional nuances such as pollen sterility (Satsuma), self incompatibility (Clementine tangor).
The actual breeding process involves removal (emasculation) of the pollen bearing anthers from the intended seed bearing flower before any pollen is shed(dehiscence). And protecting the flower from bee visitation, or accidental pollen introduction onto the receptive stigma.
Pollen is collected from the intended pollen donor flower(s) free of contamination by extraneous pollen. This pollen is transferred onto the stigma of the emasculated seed flower. The stigma should produce a sticky film on it's surface when it's receptive to pollen. The pollinated flower is then identified and protected from additional unintended exposure to pollen until the stigma is no longer receptive.
Not ever breeder will follow ever step for the sake of saving time. Not every flower will produce a persisting fruit. Unpollinated flowers and buds can be removed to increase fruit set among the control pollinated flowers. Removal of pre-existing fruit may also improve fruit set of the control pollinated flowers.
The center of the flower showing the miniature fruit (ovary) behind the style and the small round stigma. The pollen grains germinate on the stigma and the sperm cells grow through the pistil into the ovary. Fertilization occurs in the ovary. Encircling the ovary are a ring of filaments topped by anthers. In natural pollination the pollen is transferred from anthers onto the stigma.
In controlled pollination unintended pollen is excluded from contact with the stigma, while the desired pollen is placed on the stigma by human intervention.