In a simple manner, you should follow the following steps:
1. Have a place to register existing plugins, like registry sub-tree. Or you might search for plugins in specific directory (like your working directory).
2. Have you program look for the plugins that it might have and load it (LoadLibrary).
3. Have you program call required functions from plugin DLL, like initialization, functionallity, etc. (GetProcAddress)
4. Write a DLL that implements and EXPORTS the functions, that the main program is going to call.
5. Either place that DLL in a working directory, or register it in the registry, depending on the method you chose in in Step1.
This is a framework of app-plugin structure, so feel free to improvise as you move along
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When you do it, do it right.