I wanted to try the latest version of LuaTeX, so I downloaded the official precompiled binaries but these raw binaries are useless without a
luatex.fmt. How can this file be created?
I'm assuming you have TeX Live installed, so will have the file 'luatex.ini'. You need that (i.e. copy to where you are going to generate your format file), as it specifies how to build the format. You should then be able to do
to build the format file.