Well, before desktop publishing, by phototypesetting [1], before that by hot-metal typesetting [2], before that, by hand. Nowadays, with software like Adobe InDesign, of if you happen to be a CS/math/physics nerd, with LaTeX, which has a famously high-quality layouting engine that utilizes the Knuth–Plass line-breaking algorithm [3]. Indeed it's fairly well known that Donald Knuth created TeX because he wasn't happy with the phototypeset proofs he received in 1977 for the second edition of The Art of Computer Programming, finding them inferior compared to the hot-metal typeset first edition.

[1] https://en.wikipedia.org/wiki/Phototypesetting

[2] https://en.wikipedia.org/wiki/Hot_metal_typesetting

[3] https://en.wikipedia.org/wiki/Knuth%E2%80%93Plass_line-break...