If you are new in fashion designing, what you have to learn is to draw Model Sketches for Fashion Design. That is true, for model in fashion designs indeed support the perfectness of the design. Therefore, you also have to find out what you should do, and what you may not do.
As you are new here, then, you have to learn to draw croquis which offers simple pose. The reason is simple: for it is easier to draw, and the pose will not attract much attention. Moreover, if you still find difficulties in drawing, you can always trace the model from magazine to be used as the Model Sketches for Fashion Design. That is indeed practical. However, if you are tracing a model, then, make sure the model is wearing tight clothing. Next thing you can have as consideration is to trace the model who is wearing the similar garment as the idea that you have. Lastly, avoid unnecessary details on your figure, and add texture to make your design real.
Now that you know, you have to practice it. Later if you do not feel convenient with your own Model Sketches for Fashion Design, you can take a look at the well-known designers and take their models as examples. Good luck!