A character sketch is a picture in words, a way to introduce someone by explaining important information about that person in writing instead of with a photograph.
Your first decision is to choose the character about whom you are going to write. Once that choice is made, identify the important characteristics you want to include and find examples to illustrate how these shape the character's personality and/or actions.
If you want to write about Jing-mei, for example, you might start with a brief summary of the facts: she is the only child in a Chinese-American home and resents being pressured by her Chinese mother to excel in areas about which she doesn't care. Jing-mei is determined to do the things that matter to her, not to fulfill her mother's wishes to have a daughter who is better than the daughter of the mother's friend.
I looked at my reflection, blinking so that I could see more clearly. The girl staring back at me was angry, powerful. She and I were the samae. I had new thoughts, willful thoughts - or, rather, thoughts filled with lots of won'ts. I won't let her change me, I promised myself. I won't be what I'm not.
You could then provide examples of situations in which Jing-mei and her mother argue about the piano lessons, descriptions of how Jing-mei sabotages the lessons and practice times, and how the conflict between the two escalates through the story.