Your mileage may vary depending on exactly what kind of plugin you want to build.
So let's assume you want to build for at least ElectricFlow 6.0.5 a codeless plugin (basically just a collection of procedures).
Create a new repo in your SCM
Clone https://github.com/electric-cloud/PluginWizard and follow the direction in the README and the wiki for that page.
If you're trying to build a community plugin to go to our organization at https://github.com/electric-cloud-community, so the open is open source and can be picked up by the EC Community, hold on a little longer as the process is being flushed out.