In order to manually test an extension, whether it's a plugin or a declarative extension, it must first be installed into the ReSharper Platform. Once installed, existing files can generally be updated by copying a new version without re-runnning the installatione. This is a similar pattern to Visual Studio's VSIX development model.
The steps to install locally are straight forward, but unfortunately, longer and more manual than we would like. We hope to improve and automate some of these steps going forward. To install locally:
Install ReSharper to an experimental instance of Visual Studio(optional, but recommended).
Package the extension into a custom NuGet package.
Install the local copy of the extension into ReSharper, using a custom extension source.
Modify the project so that the plugin files are automatically copied when the project is built.
If after following these steps, the extension doesn't appear to load, please check the Troubleshooting page.