Lithography, usually - still widely in use today. Drawings were made or transferred to an etchable surface (initially limestone, then metal) using an etch-resistant substance. Then etching agent (acid, usually) was applied to the surface. Everything was washed and voila, a plate was produced which had a positive image which could be inked and pressed just like letters. By 1938, offset printing might have been employed, which is basically the same thing but with a rubber drum as an intermediary between the plate and the paper.