Import the build.gradle in Intellij. Run the Gradle "setup" task in the mcp folder in IntelliJ - you may need to select View > Tool Windows > Gradle to get this option. This will generate the ...
Download the minecraft launcher. Install and run version 1.12 of the game Download minecraft server version 1.12 and copy the jar file into the jars directory. Rename it minecraft_server.1.12.jar Run ...