Click on the above link. You should see a file called "EditLinkOnOpen.safariextz" in your download list.
2. Double click to install in Safari
Once you double click the downloaded file, Safari will prompt you with a message confirming that you want to install. Click the "Install" button and the extension will be installed.
You can confirm that the extension is installed by looking at the extensions preference pane:
Control clicking (or right clicking) on a link in Safari, the following menu will appear. Select "Edit Link On Open..." and you'll be prompted to edit the URL before loading it.
Let me know if you have any problems with using the Edit Link On Open or if you have suggestions for future versions.