Start visual studio and create a new project
then select Class Library
The first thing you got to do is to add a reference to Syn.Engine.dll file.
The easiest way to do so is to just right-click your project name and then click on Add Reference
And after doing so click on Browse
and browse to the installation folder of Syn Virtual Assistant and select the file Syn.Engine.dll
Click Ok and that's it. You have added a reference to Syn.Engine.dll and now you may continue with your Plugin development.