Hi,
this is by no means a comprehensive list of links/pointers but here is the reference wiki:
https://www.slightlymagic.net/wiki/Forge_APIThis looks very detailed as well:
https://www.slightlymagic.net/forum/viewtopic.php?f=26&t=19637Then there's a card-scripting channel on the Forge discord server, the link to which you can find in the main release post here, at the end of "Announcements":
https://www.slightlymagic.net/forum/viewtopic.php?f=26&t=29077(There's highest discord access verification settings, you'll be asked for cell phone verification when joining. To keep it relatively cosy

)
And finally, I might be wrong about this: Maybe keep your testing environment for custom cards offline

IIRC if custom cards aren't working, Forge reports/logs it in a dev channel as feedback for debugging purposes, maybe.
Thing is, you will produce a lot of errors in the beginning, I'm guessing. At least that's how my trial and error process would look like

Good luck!
K.