So first I built a spaceship.
Photo against black backdrop shot in my backyard
The fuselage is a tapered slice of a staircase newel post a little less than a foot long. The nose spike is a golf tee, the rockets in back are spent CO2 cartridges I collected for months, and the windows are solid plastic balls back-painted with a fluorescent paint. Little round-headed nails made good rivets. I wanted it to have a deliberately home-spun look, as if a movie propman on a small budget threw it together from junk lying around his bench.
Same picture "flopped" left to right and cropped to provide a black background
I also wanted a billowing cloud of exhaust shooting out its rear, just like the ol' Flash Gordon ships. For that I found a photo of an actual rocket launch (seemed appropriate) and cropped out everything but the smoke:
I made stars very simply in Photoshop, scattering white spots of different sizes on a black background and then blurring the heck out of them to get that old-timey unfocused outer space look. I also found a nice Apollo-era Moon photo (Apollo 14, I think) with a crater so fat, bright and lumpy it almost looked phony. One of the minor themes of WHTTWOT is that the creators of old sci-fi films and comic books got a lot of it right, so my little in-joke here is that my 1940s movie serial provides an amazingly accurate preview of the lunar surface no one would see first-hand for another couple of decades.
Then I put the pieces together. I made the exhaust semi-transparent so stars showed through. After flopping the spaceship to move from left to right, I had to do the same to the Moon photo so its shadows would be consistent with the light source on the spaceship. Here's an intermediate step:
The lunar crater isn't flopped yet: the light hitting it is coming from the right, while the light hitting the spaceship is coming from the left. I fixed that in the version below.
Since it's a black-and-white serial I converted the image to grayscale and back. Finally, to smooth out the cut-and-paste look and make all the assembled bits seem more like they belonged in the same universe, I washed out and blurred the entire image (I guess the movie projector is just a little out of focus) and overlaid a faint transparent sepia tone over the whole thing.
I did a few of these composite images. One of my goals was to make them look fake--I wanted them to look like bad special effects from the era. Then I drew my characters watching these "scenes" on screen, distorting them to account for perspective when needed. I got good mileage out of that spaceship, using it for both photo composites and as a model for actual drawings.
This was fun.