From what I remember, it was done by actually cutting and pasting. I'm not an expert but I've seen films about this being done ... so this is probably not accurate, but close to the process.
First, they would decide on the colors,if any, and the fonts for headlines and the body of the text. They would layout the columns and set the type (in a huge tray ... individual metal blocks for each letter). They would pick the photos and drawings (in fashion mags, there were mostly fashion illustrations, not photos). Then they would print the individual pieces and, finally, cut and past each piece into place on the layout, until it satisfied the editor. Then they would reset the type with the final arrangement and etch the pictures into the plates. Then the could and set up all up for printing ... in a printing press.
The plates and printing press are pretty much the same today ... bigger, and typesetting and etching is done by computer ... not by hand.