Easy way:

1. Go to WP Admin – Plugins – Add New.
2. Search for “CF7 Form Submission Limit”.

3.  Install the plugin and then Activate it.

Manual way:

1. Download the plugin zip file.
2. Extract it.
3. Upload the plugin directory to wp-content/plugins
4. Activate CF7 Form Submission Limit plugin from WP Admin – Plugins