What is a style sheet?
In general, an editor works using a standard style guide. I keep three on my desk: The Elements of Style by Strunk and White, The Chicago Manual of Style, 15th edition, and Barron's Essentials of English. When I'm not sure of how something should look, for example,
I check the style guide that I believe will fit my client's audience best. (I also have style guides for Australian and British usuage, but those are not on my desk. They rest on a shelf a few steps away.)
Sometimes, especially in technical writing, there will be other considerations. For those, an editor makes a style sheet. This is a place to keep track of how to spell special words, or how to format things like tables, captions, and headings. This is how the writer and the editor make sure the book is internally consistent.
Because I work internationally, I often help a writer decide which English conventions to use, American or Canadian. A warning if you are an American using The Chicago Manual of Style, 15th edition to alert you to Canadian conventions: There are errors in it. The Editor's Association of Canada is drafting a response giving the corrections. Hopefully the next edition will be more accurate.
If you are ready to have your work edited, you can submit a writing sample here.