If you want a good product, work with a designer.
You could spend hundreds of hours learning how to work with a program and how to optimize the workflow.
Just imagine a designer asking how to start doing the backend of the game. Sure It can be done, but I am pretty sure you would think the same recommendation. Look for someone that already knows what needs to be done.
- You need to think in terms of individual reusable assets.
- Zones of Grass, water bodies, desert
- Trees, another type of trees
And so on.
Define the style.
It is not that simple as posting the example, you need to understand it, the logic, the overall roundness, the corners, the thickness, and proportions.
Take the tool and draw.
- Use layers to separate and organize.
Here is a quick process of one of the assets.
All these are really simple shapes. Draw one.
Add another as you need for the basic volume.
Now you have your first asset. A mountain without snow.
Duplicate it, change colors and put them on the top. Now you have a second asset. A mountain with snow. (5)
Duplicate it, play with scale... Now you have your third one. A group of mountains.
But as I said, to choose what style, colors, etc you need to experiment. Here are some styles of the same asset. But there can be hundreds more; isometric perspective, circular base, more round, more faces, more everything.
But again. This can take a long time depending on the complexity of the map. It needs proportion, logic, dimensions.