Dip a cotton swab or a small, pointed paintbrush into rubbing alcohol. Gently wipe the cotton swab or paintbrush against a soft cloth to remove any excess rubbing alcohol.
Stick the cotton swab or paintbrush into the yellow anthers in the middle of the flower. Spin the cotton swab or paintbrush gently in circles to shake loose as much pollen as possible. Self-fertile, the pollen will land on the flower's protruding stigma, which sits in the midst of the anthers. You can also lightly brush the paintbrush or cotton swab laden with pollen against the stigma several times to ensure pollination. Once the pollen falls on the stigma, pollination should take place.
Remove the cotton swab or paintbrush from the flower and repeat the action on another one of the plant's flowers. Continue to pollinate each flower that the cactus produces to guarantee ample fruit production.