This process walks through creating a custom subassembly using the Autodesk Subassembly Composer.
Before beginning this process, it is recommended to attempt to utilize the stock subassemblies that are shipped with Civil 3D. The vast majority of situations can be handled with these stock subassemblies. If the stock subassemblies have been studied and will not fit the needs of the corridor design, then please follow this process.
Reference:
Refer to the following videos for more information
1. Preparing to Create a Subassembly
2. Creating a Custom Subassembly
3. Importing a Subassembly into Civil 3D