Today, companies like Comicraft and competitors like Blambot make fonts that letterers can download and then work with in programs like Adobe Illustrator.
Most letterers used an Ames guide like this one to create their lines, which meant each letter hit the same height and, more likely, hit middle in the middle of the line.
Then the letterer then takes the script and then comes in and puts all those word balloons and all those captions, and when all five of those people touch the page you end up with a complete comic book.