install MacPorts for mac, then run sudo port install i386-elf-gcc, then modify the commands in build.sh to match i386-elf-XXX, not /usr/bin/i386-elf-XXX
For linux: https://github.com/mell-o-tron/MellOs/blob/main/A_Setup/setup-gcc-debian.sh, RIP, but it takes ~20 mins on a bad pc
then run sh build.sh and qemu-system-x86_64 image.img