How can i create nicely curved edges and corners?

i m trying to simulate furniture and stopped at designing bed mattress and I cannt find anyway to do it , I ve tried designing it by subtractive brushes but it wont work . anyone can help please ???

If you’re trying to do anything beyond simple prototyping object, which is what BSP’s are intended to be used for, you would be better off using a modeling application like Blender (Free), 3DS Max, or Maya.

Those applications are designed for creating 3d assets and can do it more easily.