To be able to decompose a feature, you need to learn how to build the software in your head before you go to paper. Consider a large billing page, for example.

Step 1 is to look at the front end. What components do I need to build? (React might use components, or a set of partials and templates in rails, for example.) So, then, I know I need X subtasks, where I will build the pieces I need to expose that behavior to the client to start.

I then look down into the backend. That might be a simple task on the backend, or given 20 minutes of looking, I might see problems that I'll need to tackle. The problems are their own subtasks. I may be able to group some of the front end to a single back end ticket, or I might need a separate subtask for each.

Now, I'm likely to miss something. However, that's why this is an estimate rather than a concrete task list. It's to get started so that we know, at minimum, what it's going to take.

It's not necessarily simple, but it is a skill that can be developed.