Map building is very essential. You need to first make a layout in any painting platform. Make sure to plan forward so that when you build a map. It doesn’t look weird and goofy (unless if you wanted to). You can then start placing parts following the outline of the map (that you presumably draw and prepared). After that make sure that you add details. Details are very hard to make. That’s why if you feel that adding details is hard, try researching more about the Low-Polly style. If you still want detail ideas. I recommend giving me a certain map idea you are having. Since maps are all different and include different types of added assets and structures. Like if you are building a lab. I can’t tell you too add flowers y’know. So if you want more specific ideas please do let me know what your map idea is. If you feel like making your own custom map. (like a weird planet) then you are own your own. At the end of the day building a map is very hard, if you are taking a map out of inspiration on somewhere. Make sure to review it and try to add some of the details located in your inspired map.
Edit: Always have a dummy npc so that you can compare buildings, door, windows. I also suggest looking into some blender tutorials. Because meshes make a map look way better.