This guide is for those who are interested in editing an actual trainer/berry e-Card to use with the real e-Reader. If you are capable of save editing or you are playing on an emulator it's probably easier to inject the trainer/berry into your save file directly.
your custom Trainer (ECT) or Berry (ECB) of course (can be created with this tool)
e-Reader dev tools (https://caitsith2.net/ereader/devtools.htm)
e-Card headerfix tool headerfix.zip (source)