My google search logs contain the following phrases:
dudes that swallow cum
"how to swallow cum"
cum though my nose
how to blowjob when you hate cum
how to make swallow cum
how to swallow cum
i don't like to swallow cum
what happens if i swallow cum
what happens when you swallow cum?
Apparently I am a semen-ingestion authority. The terrifying part is that at least some of these are from different people.
Well, to answer the main questions:
-You swallow it like you swallow any other substance. There is no skill. If you can eat, you can probably manage to swallow about a tablespoon of harmless, only mildly bad-tasting, hideously emotionally loaded fluid. Just fucking swallow.
-If you don't want to swallow (or if the person's STD status is positive or unknown), just fucking don't swallow. In the STD case you should be using a condom.
-Nothing at all happens. Sheeeesh.
(I never know what those girls expect the answer to be. "You gain forty pounds and you grow body hair and you get pregnant and get AIDS and then everybody knows what you did you filthy whore and you're ruined!", presumably.)
P.S.: It's spelled come, dammit.