Flower Structure and Controlled Crosses
The design of a flower will dictate how it will function in producing seed. Plants will thus have a natural mechanism for seed production. Some plants are self-pollinators because they have perfect flowers with the stamens and pistils developing in synchrony. Some plants cross pollinate because they have monoecious flowers or the species is dioecous. In other cases, the male and female parts in the same flower become receptive at different times. This promotes cross pollination. Plant breeders and geneticists can work with plants and control the types of crosses that get made. In order to do this, they need to know some details about the flowers.
For example, soybean breeders know that soybeans have perfect flowers so they need to intervene to prevent self pollination. However, they must also know that even before the flower blooms, the pistil can become mature and the anthers can begin to shed pollen. Their intervention to prevent selfing must therefore take place in a flower that is a day from blooming.
Crossing operations can be performed with the proper tools.
Here's the Making Crosses flash animation.