How to create a DLL in C# doesn't really fit into a LabVIEW forum, I think you will find different online resources how to create a DLL in C#, maybe at MSDN.
Within the DLL you simply need to make the Function Calls to the DAQmx C API, where you will find examples online, or also provided by the driver itself.
Under Start>Programms>National Instruments>NI DAQ> Textbased Code Support you will find the C Documentation of the DAQmx API.
Christian
