Installation of a script is most often done by clicking a link on a web page. One may also drag-and-drop a local file into the browser window, or optionally use the menu bar File #8594; Open File... dialog to open it.
Any file that ends in .user.js is a valid Greasemonkey user script.
When the URL of a link is clicked or otherwise navigated to ends with .user.js, Greasemonkey will intercept the loading file by presenting the installation dialog.
Note : that Greasemonkey must be enabled to intercept the loading of the file.
When navigating to a user script, Greasemonkey will open its installation dialog instead of loading the script like a normal page. A photo of this dialog is shown underneath. It displays the name and description of the script, if available, as well as the include and exclude rules that apply.
Note : Greasemonkey must be enabled to install scripts.
The Install button
This button will, of course, install the script in question. Like the Firefox extension installation dialog, this button is disabled for a few seconds to avoid the same potential security vulnerability.
The Cancel button
This button will cancel the installation of a script.
The View Script Source button
This button will allow viewing of the source code contained in the script. At this point, Greasemonkey has already downloaded the user script in question to display the name and other details.
When a user shows the script source, it displays the temporary file that Greasemonkey has already downloaded depicted in this screenshot.
*In this window there is an information bar at the top similar to the Firefox extension installation security warning. Clicking the install button here will also install the script*
I hope this helps a few people who have troubles.
I'm not an expert but this guide sums up quite a bit.