You can shorten any URL by just typing the tny.im address plus the URL you want to shorten.
Just head to your browser's address bar, type "tny.im/" before the URL (right before its 'http://' part) and hit enter. You can also use "https://" instead if you want to visit tny.im over a secure connection.
Unfortunately, this won't work well in many new browsers, including Google Chrome and Chromium, because these automatically remove the 'http://' part from the URL. Because of this, you must add the prefix above described plus 'http://' (in front of the prefix, before the current URL). This means you'll need to type more.
Here's an image explaining what you should do to shorten, for example, a Google search results page URL:
Prefix-n-Shorten can also be used by website developers as a simpler alternative to the API when only simple link shortening is needed. For example, to shorten the URL of any page, just have the user be redirected (or click a link) to the prefix above plus the long URL.