How to debug c++ dll?

I have c++ mex files around 20 and I need to compile all of those mex files using same c++ dll. I have compiled all, now I got some issues while running a script which uses those mex libraries.

So I want to debug the mex files and dll. I got the procedure for debugging mex but not dll. Can anyone please tell me the procedure for debugging a dll. I have visual studio 2012 installed on my computer and also the source code of the dll.