Hi Chuck,
That’s an overly broad and non-specific question 
I suggest you start at the Documentation Index, in particular in the section Module Creation. There, you will find reference materials, guides, and so on. Also, the section Tutorials can be useful.
Other than that, I suggest you find a simple module - say Battle for Moscow - and open it in the Vassal Editor to see how things are done. Then, when you understand what is going on, you can progress to more advanced modules, such as Battle for Moscow and see how things are done there.
For more specific questions, search this forum for past questions and - usually - answers.
Good luck with your endeavour.
Yours,
Christian