Not sure what level is difficult for you. First step is to unlock the mcu with a SWD programmer, eg from nukleo dev board or rpi.
Next is to backup the internal/external memory and to patch the original FW. Smashstack and Tim made nice tools. You will also need to convert the roms before you can upload them. As the original flash is only 1MB, it also make sense to replace it with a larger one. I used 16MB as I only care for LCD emulation, which gives much more available meomory than I will ever need. All Tiger/Konami/Nintendo LCD games together are below 16MB

A good start is to join stacksmashs discord server.