Hi there, charity bidder is here)
I have vast experience in wordpress plugins (you can see it in my profile, almost all jobs were about wordpress plugins) and I use it in majority of projects.
I would do the voting button as a shortcode that you can place in any post or page or text widget.
My process:
1. We outline all details in technical specifications in the beginning of the project to clarify everything.
2. I use a version control system (Git) that will ensure that any action can always be reversed, improved or changed.
3. I have my own development servers where your project will be always available for you to see the progress.