Shihtzu's shouldnt be fluffy, so you are good there. If you want her to look like a shihtzu, I am assuming one in full coat? If that is the case, you need to let the coat grow. When it gets long enough, the weight of the hair will make it lay straighter, though some coats just don't lay well, and still have too much body and wave to them. Personally, I think those type of coats look better in shorter trims, and are much easier to take care of. You can use an attachment over a blade to leave her body coat as long as 1 1/2 inches. You may be able to get her to look "fluffier" at a length like that if she has alot of wave in her coat.