To reflect is to consider, ponder, look back, contemplate, wonder, return, reconsider, mirror, reveal.
Take some time to look at your work and think before you begin writing in your sketchbook. Remember back to when you made it - what were you intentions? What were you trying to do? Did you achieve that result?
Begin by writing about the process of making the artwork. How did you make it? What medium or media did you use? What were your ideas?
Next write about your ideas. Many reflective writers keep in mind questions, such as "What did I notice?" "How has this changed me?" or "What might I have done differently?" Thus, the focus is on writing that is not just descriptive. (Wikipedia) Some other questions you could consider are "What did I learn by doing this?" or "What parts of the artwork do I like? What would I change? Why?"