To create a .dll, you name your source file with a .c or .cpp extension and then tell your compiler you want to build a dll from that source. The .decls is just a text file for the Blitz IDE to read.
To use Visual Studio, read the tutorial here.
http://msdn.microsoft.com/en-us/library/ms235636(VS.80).aspx
To use Visual Studio, read the tutorial here.
http://msdn.microsoft.com/en-us/library/ms235636(VS.80).aspx