After you prep your microSD card just install all the files in the latest release here:
https://github.com/hoglet67/RGBtoHDMI/r ... 21_156e2f0
Install the card into the Pi, install the Pi Zero on the RGB2HDMI Denise adapter, plug in your HDMI cable to your display, power up the A500 and should see an awesome image!
Oh yeah, have the menu button wired up also because you need to be sure the Amiga profile is being used. In addition, calibrating the display might be necessary as well if it does not look quite right.
Good luck!