Dental dams are used to protect yourself from STD’s when you go down on a girl or during rimming. Oral sex is less dangerous than intercourse, but how risky it is remains open to question. If you want to be really safe you better use a dental dam.
A dental dam is a thin six inch square sheet of latex that you spread over the vulva/anus or you can keep it in front of your mouth. It acts as a barrier.
It’s a very effective method of preventing infection to spread from the anus or vagina to the mouth.
Test it beforehand by putting some water on it to be sure no liquid seeps through.
Put some water based lubricant on the vulva or anus to increase the sensation. Use only water-based lube, because oil damages the latex (and can cause vaginal infections).
Make sure that only one side comes in contact with the genitals or anus. To ensure this you can use it only once (washing is not enough to get rid of viruses or bacteria and may cause tears not visible to your eyes).

Making a dental dam from a:
condom
latex gloves