The first move is the simplest. STOP reminds you of two things.
When you hit a page and start reading it, STOP. Ask yourself whether you know and trust the website, author, publisher, or publication. If you don't, use the other SIFT moves to understand what you're looking at. Only read it or share it once you know what it is and can verify that it is reliable.
After you begin the process and use the moves, it can be too easy to go down a rabbit hole, chasing after more and more obscure facts or getting lost in a "click cycle." If you feel yourself getting overwhelmed in your fact-checking efforts, STOP and take a second to remind yourself what your goal is. Adjust your strategy if it isn't working. Ensure you approach the problem at the right depth for your purpose.