What is being assessed
Reflective assignments confuse students because they seem to invite personal writing while being marked on rigour. What the marker is looking for is not how you felt but what you learned, on what basis, and how it changes what you would do next. Feeling is the data, not the finding.
Depth
Reflection has levels, and most drafts stop at the first. Description says what happened. Analysis asks why it happened that way and what explains it. Critical reflection questions the assumptions you brought in and what you would change. Marks live in the second and third.
A rough test: if someone else had been in the room, could they have written your paragraph? If yes, you are still describing.
Using a model
Structured models help when you are learning. Gibbs runs through description, feelings, evaluation, analysis, conclusion, and action plan. Kolb frames learning as a cycle from concrete experience through reflective observation and abstract conceptualisation to active experimentation. Schon distinguishes reflecting during an event from reflecting after it.
Use whichever your course specifies. Use it as scaffolding rather than as headings, and once you are comfortable, let the structure follow the thinking instead.
Bringing in theory
Most reflective assignments expect you to connect your experience to course material. Done badly, this is a concept dropped in at the end of a paragraph. Done well, the theory does explanatory work: it accounts for something in the experience you could not otherwise explain, or it fails to account for it, which is often more interesting.
Register
The first person is expected here. That does not license informality. Write about your own thinking with the same precision you would use for someone else's: what you assumed, what surprised you, what evidence changed your mind. Vagueness about your own reasoning is still vagueness.
What to leave out
You control how much you disclose. Reflective assignments sometimes touch on difficult material, and there is no requirement to write about anything you would rather not. Choose an episode you can analyse honestly and comfortably. The mark is for the quality of the reflection, not the significance of the event.