A typical fashion print is an engraving made on paper, usually nice and thick, with a wood block or a copper plate.
The better engravings were then colored by hand using stencils. The even better ones were colored one by one with watercolors.
It says so on the publication itself.
Of course many of these publications have disappeared, been separated or lost. In the pictures below we have a pristine example of what the lady would receive in the mail, including all the patterns and of course...a fashion print.
Now this was the short answer. Of course there is way more to it.
With advance in technology and printing techniques antique fashion prints evolved as well.
By the way, anyone having questions is welcome to ask.
P.S.: If you want a very academical history of Fashion Prints and can read French, please enjoy Raymond Gaudriault's "La Gravure de Mode Feminine en France". It is certainly the most thorough research on the subject.