that's exactly what I'd say.
Templates and Frameworks are for developers who need to save time and tedium from doing the same thing over and over again.. but have already been able to create a plugin from scratch without Template/Framework assistance.
Your best bet is to attempt to track down some open source C++ plugins and look at what they do. Study the functions that are needed by Decal and just generally see what is required in making a C++ plugin. Try not to "copy/paste" code as you should make it all by hand at least once to get an understanding of what you're doing.
make sure you also read
http://jvarga.berkeley.edu/decal/rules.htm as it is very important. It sets some guidelines for developing plugins and requesting assistance.
and just as an FYI... .NET Plugins (in C# or VB.NET) are MUCH easier than a C++ plugin. If you REALLY want to create one in C++ then good luck with that, but many of the previous plugin developers are switching to .NET because of how easy it is.