Early in the spring, when tomato plants are starting to load up with blossoms, this is the best time to find viable mother blossoms; pollination has a higher success rate in cooler temperatures.
This shows a good example of the ideal blossom—not too small or tightly closed so that it’s immature and unreceptive to pollen—but we also don’t want it partially or fully open, which would mean the blossom is fully mature and can self-pollinate.
Notice that, beneath the sepals, the petals and anther cone are still tightly closed and light green in color with no sign of the stigma peeking out—this is the “perfect” blossom. Sometimes a blossom can look very close to this but have an exerted stigma, which will allow foreign pollen to attach and pollinate the blossom, even sometimes before it has pollinated itself.
Once you’ve found the ideal blossom, prepare it for the cross by emasculating the blossom: Remove all the male parts of the blossom so it doesn’t self-pollinate before you can make the cross. To do this, carefully remove the sepals, petals, and anther cone. Some people use their bare fingers; others use a pair of tweezers and maybe even a jeweler’s loupe to see—some blossoms are quite small.
You see the blossom with all male parts removed, leaving only the stigma, style, and ovary. I prefer to emasculate two or three blossoms for good measure. At this point, most breeders will cover the emasculated blossom with an organza pouch or some sort of protection to keep it from accidentally receiving pollen from an unintended source; this is called “bagging the blossom.” It is also a good practice to tie a brightly colored ribbon on the stem of the blossom cluster to easily find the blossom in case the bag blows away. After roughly 24 hours, the stigma will become mature and sticky—it is now viable and ready to receive the pollen from the male donor plant.