Install msys2 and open a "MSYS2 MinGW 64-Bit" shell.

Run the following commands to clone and start building:

pacman -Syuu
pacman -S git
git clone https://github.com/mkeeter/guile-mingw
cd guile-mingw
makepkg-mingw -s

It will make it almost all the way, then fail when building documentation.