We don't want to reinvent the wheel - we just want it to make sure it runs safely and efficiently ;)
CARLA has a great tutorial on how to create environments - including the tools to do so - such as Road Runner.
See their documentation here (*but see note below first before clicking through and getting lost in the documentation):
CARLA Documentation on Maps
When the time comes to "Map ingestion in a CARLA package" - we've built you a tool to make it simple and easy. We've created a quick command line that will do it all for you - building Unreal and baking the assets (a long and painful process) - you're welcome.
After running this command, your map will be available in the cloud for cloud execution or for download to run locally.