Above is the original art, the studio photo, and the finished cover illustrated by Gerald McConnell for Tidal Wave. The black and white photos of the models used for some of these pulp covers were as racy if not sexier than the final art.
It would be impossible to give the prep work, that illustrators had to go through for these covers, their full due. But we'll take a run at it. First they would get the idea of the art from meeting with the editor or art director, than they would go back to their studio and start drafting models (some times other artist, students, friends or actual models). Third, it's the shoot, which starts with taking Polaroid's to get the set up, and then taking the photos, and finally running some film to get a good photo to illustrate the cover from all the takes.
Robert A. Maguire, an earlier student of Frank Reilly, was a "titan" of paperback illustrators, especially the genre of murder mystery and detective pulps. "Maguire's mastery of the "femme fatale" created a vintage paperback icon: his women are passionate yet somehow down to earth, approachable, though sometimes at your own risk." Their web site has a great collection of his work, and has gathered up some of the model shots Maguire used for his illustrations. It's a captivating collection which helps to walk us through some of the process from idea to cover. A technique similar to how Gerald McConnell and other artist of the 1950's and 1960's were able to produce two or three covers a week.