WordPress.org

This plugin is installed just like any other WordPress plugin. More detailed installation instructions are available on the WordPress Codex.

1. Download Custom Field Widget
2. Extract the /custom-field-widget/ folder from the archive
3. Upload this folder to ../wp-contents/plugins/
4. Activate the plugin in Dashboard > Plugins
5. Customize from the Design > Widgets menu
6. Enjoy. And then consider donating

In other words, just upload the /custom-field-widget/ folder and its contents to your plugins folder.