Save the file. When you do, you'll need to determine two things: its file name & where to save it.
File name: The file must end with .sublime-snippet, like this: foo.sublime-snippet. What goes in front of .sublime-snippet is up to you. I like to use the tabTrigger if at all possible, along with the main part of the scope in front of the tabTrigger, giving me this: html-p.sublime-snippet.
Where to save it: When you press Save, Sublime Text should automatically try to save the file in the right folder.
You shouldn't have to worry about the location, as Sublime Text should take care of it for you, but if for some weird reason you're not in the right place, then you'll need to navigate to the right location.