Hi, in this photo we have a direct light coming from above, at an angle of about 45° and slightly behind the girl. As it is a direct light the shadows are quite defined, especially in the folds of the dress, although on the body they are softened (probably with photographic diffusers, fashion photos use a lot of artifice). However, first you have to identify the parts that are mainly in shadow and those that are mainly in light: squint your eyes while looking at the photo, in this way the half tones will blend in with the main shadow masses and you will be able to make a clearer division. Don't focus on each individual shadow in the first place but simplify the body shapes into geometric objects so that you can apply shadows more easily. When you are satisfied with the shapes you can concentrate on minor shapes and details. I recommend that you use a softer pencil than the one you are using now because otherwise you will find yourself practically having to do a Doré-style etching job (beautiful, by all means, but not suitable for sketches) and perhaps try some simpler figures first, i panneggi sono una delle cose più complicate! ^^;