This one in particular was pretty hard to make, I essentially had to find a reference for each individual country at that time period, and then add it to the grey map of the world (which I created using Jetpunk's standard map of the world).
However, most maps are quite simple to make. The primary ways are via downloading existing SVG maps/existing featured Jetpunk maps, tracing PNG images in Inkscape and/or drawing based on reference from another map. I have gained loads of experience from hours of just messing around in Inkscape really. Some larger projects tend to take a while, it's not often I'd make a map of this sort. Feel free to ask me anything about map-making, though. I'm by no means a tech-wizard but I don't mind trying to help at all!