Flowers and the Plant Breeder
The work required of the plant breeder depends on the type of cross desired and on the flower structure of the species.
Monoecious plants do not require flower manipulation that involves removing a stamen but may require flower protection. The female flower can be protected from pollen by covering it before it’s stigma is receptive. Pollen can be collected from a male flower and introduced to the covered female flower.
These crossing operations require the plant breeder to use the proper tools and timing.
Here's the Making Crosses animation.
Dioecious plants require the least flower manipulation. For example, if female plants are grown in a proximity to only one type of male, all offspring produced on that female are the result of pollen from the same male. Controlling crosses can be as simple as deciding which plants to put in the same greenhouse or which plants to place next to each other in the field.