Answer
There are two fundamentally different paths, and it's worth being clear which one you actually need: Configuration, done entirely through the standard IFS application UI (via Custom Projections) without developer access — the major limitation being you can't write custom PL/SQL methods this way, only expose/configure existing data; and Customization, which is developer-oriented, requires IFS Base Server knowledge, and is done through Developer Studio — this is the path that lets you write genuinely new custom methods. IFS's Configuration and Development Guide sections of the technical documentation cover each path respectively, and IFS Academy has dedicated courses on the custom objects framework if you need more structured guidance than the docs alone provide.