Thanks! It's without a doubt a pretty painful process. I'm using d3.js (
https://d3js.org) for projecting a vector representation of a specific area. There is a lot of great tutorials / videos out there so a good starting point is probably just to search for e.g. "d3.js maps".
D3.js might seem overwhelming at first, at least it did for me, don't let that discourage you.
There are many vector sets where existing countries are already present but in my case I have to plot the highlighted map areas on my own. For that I use QGIS (
https://www.qgis.org/en/site/), then just save those areas in a vector file and project them onto the map.
I guess that's the basics.
I have spent many, many hours generating/mapping the background image, pruning the data files from redundant information, repositioning tooltips after a zoom, etc, but perhaps you don't have to, so cross that bridge when you get there.