Download the newest version of the MSYS2 installer from here and install it.

Run the MINGW64 prompt if you wish to build a 64-bit version of the executable, or the MINGW32 prompt otherwise.

Enter pacman -Syuu in the prompt and hit Enter. Press Y when it asks if you want to update packages. If it asks you to close the prompt, do so, then restart it and run the same command again. This updates the packages to their latest versions.

Enter this command to install packages necessary to build sm64ex:

pacman -S unzip make git mingw-w64-i686-gcc mingw-w64-x86_64-gcc mingw-w64-i686-glew mingw-w64-x86_64-glew mingw-w64-i686-SDL2 mingw-w64-i686-SDL mingw-w64-x86_64-SDL2 mingw-w64-x86_64-SDL python3

git clone https://github.com/sm64pc/sm64ex.git -b nightly
cd sm64ex

For each version (jp/us/eu) that you want to build an executable for, put an existing ROM at ./baserom.<version>.z64 for asset extraction.

For example, if you want to build the US version, there should be a ROM file called baserom.us.z64 in the sm64ex directory (meaning next to the Makefile).

If during the build process you get messages saying that the ROM has an incorrect hash, there is a possibility that it's a V64 ROM that needs to be byteswapped. To do that, use this web tool.

make EXTERNAL_DATA=1