You will want to do this in mid-April. If you wait until the end, you may not be able to apply the feedback to another draft. You should search for big points you want to develop. This might be a brainstorm, but get these ideas out and then try to formulate several major points. A major point is one that is sufficiently big that it requires development.
Development is done through explanations, examples, supportive facts and details.
Importantly, make these as concrete as possible. One common symptom of poor reflective reports is that they make very generalized statements without any concrete examples or factual support related to a real context. Finally, I recommend that you send me at least one draft to get my feedback. You will want to do this in mid-April. If you wait until the end, you may not be able to apply the feedback to another draft.