Technically speaking, an author's style is created by the words used and the pattern of the words used. Tone, the words chosen by the writer to create a given feeling in any given piece of literature, and sentence patterns and the variety of said patterns work together to create what one would call the author's style.
But style is more than words on a page. Style is the attitude which surfaces, consciously or subconsciously, as a writer puts pen to paper. Style is something which words can only hope to accurately depict. The words do not make the man, the man makes the words. The character of the man is what drives those choices. One could argue that author's style really has nothing to do with the words and sentence patterns, but really only with the heart, mind, and soul of the writer. It has nothing to do with grammar and usage, but has everything to do with what the writer feels and believes about any given subject presented to them.
When thinking about style, first consider the clothes people wear. Do you know someone who always dresses in black or who never wears plaid? Maybe the clothes are pure vintage (from the local thrift shop, perhaps) or very modern and straight from the most fashion-forward stores. Whatever they wear, people choose clothes to express themselves and their individual "style."
Authors have style, too, but it's called literary style or the writer's voice. Writers express their ideas in specific ways by using just the right word and putting it in just the right place. A literary style reflects the author's personality and the purpose for the writing (that is, the idea to be expressed). For example, you might have two writing assignments — a short story for 10-year-olds and a research paper for your doctorate degree. The purpose of each assignment requires a different tone, word selection, and sentence structure. Which one would be casual and conversational? Formal and academic?
Other elements that contribute to writing style include the rhythm of the language, figures of speech, punctuation, and character development.
When you analyze author's style, you need to consider the author's point of view, sentence structure, syntax, and tone.