First, you sketch the AXES, meaning that at the beginning you should sketch the human body in a few lines. There are a lot of positions in which the human body can be represented but in short, you must observe the lines of the shoulders and hip.
If interested in studying the human body, here is a tip and what I do: search for fashion magazines to inspire you. Carefully observe the curves of the human body and imagine the lines behind the image.
The rule is simple: when the line of shoulders is up (ascending), the line of hips goes down.
Secondly, after drawing the AXES, you will draw the PRIMARY SHAPES: you decompose the body into cylinders, spheres, cones, cubes and so on.
Thirdly, you will go into ANATOMICAL DETAILS. You will imagine a “skin” covering all primary shape elements and covering them in smooth shapes.
This is the system you should use every time you want to draw fashion sketches.
Use fashion magazines or plastic mannequins to get inspired when it comes to poses. Copy the image in your computer and mirror-rotate them to have even more positions to study how to draw the human body. If you want, yu can take some photos, make them black and white and try to figure out the main axes of her body. Observe the tension of every line and simply memorize some positions in order to use them later, when drawing fashion sketches. For the moment, you can study from my videos or from some of my sketches and photos on my fashion teaching site.