Corridor in Civil 3D

I am working on a revitalisation project of a small river in my school. I have to design a compound stream channel. I have prepared the surface area, alignment of the small riverbed and the wall river bed, banks and the bases of slopes. I don't know how to do assembly and from than the corridor constrained by the lines I have already drawn. Could you please help me with that? All the materials are prepared and only the assembly and corridor needs to be done?