Hello Andres!
Welcome. Since this is a new post and deserves its own replies, I’ve moved it to a new topic.
The best (and really only) place to start is by doing a trace in the regular UI. Many use the nice Trace Helper Utility
Trace Helper Utility for Epicor ERP 10 - ERP 10 - Epicor User Help Forum (epiusers.help)
This will show you what fields are populated in the standard product. Then you can design your customizations to duplicate that process.
Mark