How exactly does vote fraud occur?

Well, there are dozens and dozens of ways. - John F. Di Leo

Maybe even hundreds (since I've been studying the issue for decades, and I'm always still discovering people committing methods that are new to me)...
The Washington Times has an interesting report today about a California Democrat who pled guilty to a raft of crimes in this arena, to be summed up as follows:

She'd be recruited as a paid petition collector or voting registrar, meaning that sometimes she'd be hired at a flat fee per name to collect signatures or to get people to register to vote..... if they weren't already registered when she asked them to sign a petition, she'd take care of both issues the same day.
One of her favorite methods was to troll the various Skid Rows of Los Angeles and its vicinity, looking for homeless people. 

If they didn't have an address, she'd let them use one of her own addresses.
This way, since California likes to mass-mail ballots to registered voters, she would get lots of ballots delivered to her house - and yes, she could vote them all...