Once you've downloaded the browser extension, a 'W' button will appear in the top bar of your browser. If it doesn't, make sure it's enabled in your browser's extensions menu.
Then, when you're on page you want to save, click the 'W' button. If you're not logged into Wakelet it will ask you to log in. Then you can save the page directly to your items, into an existing collection or into a new collection.