First things first, you'll need to provide a copy of a clean, headered1 Super Mario World ROM (USA version) renamed to clean.smc and placed in the "resources" folder so it can be found by the build tool. Take care to ensure the file extension* is .smc and not .sfc.
It is important to do this step otherwise you can't get started.
*this may require you to turn on extensions in Windows' File Explorer.1If your rom does not have a header–which you will notice because tools will be unable to work with it–open it in Lunar Magic and you will be prompted to add a header. Hit "OK" after being prompted but DO NOT SAVE the rom.
To start using the baserom you will first have to initialize your project as it comes without any of the tools bundled. The baserom is cross-platform and depending on which version you downloaded there will be different setup scripts provided.
For more detailed information about the setup process you can read the Baserom Setup documentation.
If everything has been set-up correctly, and the setup process went smoothly, the baserom is ready for use!
You can jump right into working on your project by running "callisto.exe" from the tools/Callisto folder and select "Edit" to launch Lunar Magic. From Lunar Magic you can access much of your project with the custom User Toolbar.
Important: don't run the baserom's tool's individually. Use Callisto to make all changes your project.
If you have further questions you can read the FAQ or Basics pages or visit the #baserom-support channel in the Romhack Races Discord server.