Here is an example of how I write a character sketch:
When Norman Mavrich appeared in town in his ranch clothes and with his cowboy stride, many were the admiring glances sent his way. He was not what many would call handsome, not having dark hair and eyes nor a high brow and a finely cut face. In fact, if you got a good look at him, most would call him plain and ordinary. He was tall and lanky as most of the ranchers in the area were, yet there was a look about him that was different somehow from the average rancher. His hair, which was usually covered by his hat, was brown and, much to his disgust, curled. When he saw a wrong, his gray eyes, usually so kind and sparkling with the joy of life, would suddenly turn to steel and flash with sparks of fire while his jaw became set and his lips firm. His strength had stood him in good stead a time or two with some hot head, though he much preferred to use his skills as a diplomat to settle things than his fists.
Note from Mrs. M: Not everyone chooses to create a character sketch in the past tense. You can (like me) create character sketches in the present tense and more "note like" for your own records, a little like my character biographies. It all depends on if you plan to use the actual sketch as part of the story or not.