Making Cosmoteer-style visuals
Basics
Get some decent drawing software. Many are available for free on the internet (GIMP, Krita...) and others are specialized for this type of graphics (Aseprite).
Dig into the game's files, for example in Data/ships/terran/, where you will find many reference images.
Copy the ones most closely related to what you want to make and put that into your separate work folder outside of the game's folders.
Import them into your drawing software for, well, reference.
Make liberal use of layers to minimize the impact of any mistakes.
Be patient : a day of drawing per part is probably expected if you use a competent program and know how to read and understand the sprites.
Roofs & Normals
Three possibilities :
1 - Draw them manually or "stitch" vanilla normals (this means copy-pasting parts of your reference images to seamlessly assemble them together into something new).
2 - Generate them badly with modlab.
3 - Combine both methods for best results.
Copy the colors from your reference images and draw with those colors.
For this, use the "Pick color" tool of your drawing software.
Advanced
The edge emptyness of a sprite has the normal of corridors. Edges sometimes are standardized too.
Original text courtesy of JaniTNT on Steam/Discord.