It should be a ton easier now with xLights 2016 than with 3.x/4.x. If you look in the xLights/effects directory, you can see that each effect now has it's own c++ class that implements a few methods. Thus, you pretty much need to implement that class (actually, its two classes, one for the effect and one for the Panel in the UI, but the latter is mostly generated via CodeBlocks), add the class to the list in EffectManager.cpp, and that's about it. The only two "existing" files you need to modify are the EffectManager.h and EffectManager.cpp.
Anyway, I'd look at one of the simple effects in there to use as an example.