First of all, you will need to have cc65 installed, which will depend on the OS you use. Please see the website of cc65 for more information.

When you use Linux or macOS, I have made a Makefile, which may make life easier. Just run

make default

in the rom directory and you should get a bios.bin file (32 kbyte) which you can burn into a 28C256 (or equivalent).

If you prefer to do it manually, see the commands in the next code snippet

cl65 -t none -l bios.lst -o main.bin main.asm
cp main.bin > bios.bin
cat main.bin >> bios.bin

I copy the rom twice into the final bios.bin file, as I have not (yet) found a way to burn a smaller binary at a specific offset with the Linux minipro command. 

I have included (or try to include) iBOM HTML pages for all boards, as to make it easier to find what to solder where