I'd tackle this project like this:

- Create a list of all potential buildings and maps you think you will need
- sort your list by categories like buildings, terrains, dungeons, etc. so that you have a better overview of everything that you need
- under each category, find things they have in common to later create a base map
- If you're familiar with creating maps, choose you tool that you like to work with and start creating
- If you like to use specific software for creating maps than i'd search for that and start creating afterwards

If you will create the maps by yourself e.g. with Procreate or Photoshop i can recommend different brush sets (some are free, some you have to pay for):
https://www.mapeffects.co/
https://kmalexander.com/free-stuff/fantasy-map-brushes/

There are also a ton of great YouTube Channels. Here are some I've watched recently:
Map Effects
Maps by Owen
JPCoovert
WASD20

Hope I could help