In the ECTs' account, head to the "Early Career Framework" page, where they will see listed the modules you will have to complete for the programme. (As a Mentor, you can also see this from their "Workspace" when you access it from the "My ECTs" page.)
In each module, they will see a list of the self-studies. The first one is always a core self-study, and then the diagnostic for each module.
By clicking "Start diagnostic", your ECT will be able to complete the assessment. The output will be a recommendation of three self-studies to select. ECTs are free to override this choice if they want to, but the recommendation takes into account answers to the diagnostic questions.
The ECT only has to complete at least three self-studies per module, but can still choose to complete more. The ones chosen will be marked with the "Selected" tag, as in the screenshot above.