The diploid plant A has a cytoplasm, which is genetically different from the cytoplasm of the plant B. To study the cytoplasmic-nuclear relations you need to produce a plant with the cytoplasm of plant A and nuclear genome of plant B.
To produce such a plant we need to remember that the cytoplasm is maternally inherited. So plant A is used as the mother and plant B is used as the father to produce progeny. The progeny is backcrossed using plant B as the paternal parent. So the plant B has been used as a father for two generations. This procedure is repeated until the whole genome is from plant B.
Then you will have a plant with the cytoplasm of plant A [which acts as a mother] and the genome of plant B [which acts as a father].