Making a custom Amstrad 464 and 6128 plus & GX4000 cartridge is quite simple – but it does mean sacrificing an existing cartridge. The main reason for this is you need the ACID protection chip. You can read more about the ACID chip on the CPC Wiki.
I sacrificed my fake SwitchBlade cartridge – but I successfully removed the EPROM so I can still use it in my ‘Multicart’.
So, all I had to do was removed the old EPROM and fit a IC socket.
Once the cartridge was ready, I needed to burn some software onto a blank 27C1001 EPROM. The CPR files you find on emulation websites etc will not work. You need to convert them to .BIN binary files. There is a great tool here that does that very job. So, once you have the .BIN file, write it to the EPROM using your EPROM programmer and you are good to go.