Really watch out for the timing of this. EPQs have two submission dates per year, and schools have to have time to finalise their grading before submitting them.
We thought my daughter had a year to do her EPQ, doing it in lower 6th. So she was thinking a little about it over the summer but not much. Starts 6th form, figures a topic, starts working on it. Suddenly I'm hearing stuff about drafts over Christmas. What?? Turns out they had to complete by Easter for a ? end of May submission. If I'd had any inkling that their "year" was actually 2 terms I'd have pushed her to think more actively about a topic over the summer and perhaps intervened earlier when she did start her topic and got bogged down (intervention was, for goodness sake watch the film first then try to read Shakespeare). I did feed back to her school about informing parents better...
Come to think of it, must check with DS's school where he's about to start an EPQ. He's got even less idea what to do than his sister had (though I could be wrong), and I may need to encourage him to think about what is achievable when he knows just what time he's got to do it in.
I agree though that the process part of things attracts a good proportion of the marks. Encourage them to record their ideas and thoughts systematically from the start.
Following up hobbies, interests, cross-curricula things, stuff they might want to do at uni, all a good start for topics. Then narrow down, and narrow down some more - but make sure this process is recorded as it can contribute valuably to the assessment! My niece got an A* doing one last year (started it in the first lockdown once GCSEs weren't happening). Her initial idea was great, then she narrowed it down to researching a smaller number of people, then finally focussed just on one person but referenced the relevance to others. That focus was essential to being able to complete a coherent piece of work.