How can people practice safe sex during oral sex on a woman?

None, ... Contacting the right area with bacteria will always brings the potentially offending germs to the obviously or microscopically injured cells of skin or mucosa incurred by sexual acts. Hence there would be no imaginable ways to lessen STD risks; this is life reality and prevention by not doing it is the best way to avoid. Nonetheless, the odd of catching STDs is still low, but not impossible.