The SN.exe tool can be found under the SDK's folder, under C:\Program Files (x86)\Microsoft SDKs\Windows. You can simply open the folder and search for sn.exe and pick the latest based on what version of .NET you have. For example, it's C:\Program Files (x86)\Microsoft SDKs\Windows\v10.0A\bin\NETFX 4.6.1 Tools.
Note: If you use the Developer Command Window, the path will already be loaded.
Simply navigate to the folder where the DLL is and enter:
sn -T <ddl name>.dll
sn -T MyNewDLL.dll
* If you don't know where the DLL is, you can find it under the bin\debug or bin\release folders in your Visual Studio project.