Masking and Stenciling:
While working with soft pastels because of the dusty nature of the medium it makes it difficult to create clean and hard edges. With the help of masking techniques one can create sharp edges using stencils. For the sharp straight line you can use paper edge to mask the line stroke.
 

For masking the line or shape, cut the shape out neatly on a piece of tracing paper and stick it on the drawing surface using art tape. Stick the mask neatly on the surface and then add the color on top of it. Clean the extra dust and peel away the mask carefully to reveal the crisp shape underneath.