Reflection exercises help you understand why you do the things you do in the design process. They help you articulate your design decisions and observe the interesting ideas and lessons that are a by-product of UX activities.
Reflection is an incredible way of fostering self-awareness and it’s a very smart way of documenting your work and progress, making it easier later down the line to create a portfolio or articulate your value proposition and definition of self.
The easiest, most accessible way of doing reflection work is journaling. Whenever you have some free time or at key moments throughout the process, sit down with a good old pen and an agenda or diary and start jotting down your thoughts, observations, interesting ideas or absolutely whatever is on your mind in regards to the UX work you’re doing.
You can find an interesting method in this article and read more here.