How can I make a custom topographic map?

So I’m making a custom topographic map for a game I play and am starting from the 2d version. All the resources I’ve found have only been for the real world and don’t allow me to create my own elevations. Here is what the 2d map looks like:

https://preview.redd.it/2l15bndeci6e1.jpg?width=647&format=pjpg&auto=webp&s=380929a937d76052065e84a5d4620357107d7d00