The prerequisites you will need are having Xcode, the Xcode command line tools, Homebrew, and CMake. The command line tools can be installed by running xcode-select --install after installing Xcode itself, and CMake
can be installed through Homebrew by running brew install cmake.
Previously I had you clone Sappharad's fork of the Mania decompilation for you to do this. However, that hasn't been updated in a while, and I don't feel like bothering him enough to update it. Therefore, you can use my fork instead!
git clone --recursive https://github.com/MarioMastr/Sonic-Mania-Decompilation