This video from the University of Hull gives some good advice, but it is not explicitly for CAS.
You can also use the seven learning outcomes to help you write reflectively. Perhaps focusing on just one, or two, of them.
The DIEP model for reflective writing stands for:
D - Describe
I - Interpret
E - Evaluate
P - Plan for the future
The following video by the University of Melbourne explains this model and shows that reflection is valued beyond school.
Another way of structuring the reflective process is by using the Gibbs reflective cycle. This video from Expert Program Management is job-based!