RE: How to become a developer for D365FO
I want to emphasize the same things than others who already replied.
First of all, you need a functional D365FO expert to help you get most of the system, and help you utilize the existing processes and features. And, if there are functional gaps and a customization is needed, you still need the functional expert to define how to best augment/integrate with existing processes with your customization.
Second, succesful development for D365FO requires good understanding of the system, too. The developer should understand how to utilize existing business logic, as well as implement the customizations in understandable, maintainable and reusable way. In addition to development, someone needs to be able to set up and handle the ALM (Application Lifecycle Management) part such as source control, builds, deployments and so on. It can take months, even a year before a fresh graduate without previous D365FO experience would become a productive contributor.
You will save a lot of money and trouble if you hire some functional and technical experts, at least to help you start with your journey.