In some cases you won't see the page you wanted to get, then you simply have to navigate back to it within the screenshot box. Please note that you may also enter a URL in the navigation bar on top of the screenshot box (this may even be a different website).
3. Take a screenshot (Win+PrtScr on Windows - Shift+Cmd+3 on MacOSX). Your OS should save an image file on your local computer.
4. Open the screenshot file and crop it, using the guidelines and cropmarks on the image. Most simple image tools offer a crop feature (Preview on MacOSX for example). You may also use a fully featured image manipulation program such as Adobe Photoshop of course.
5. Save your cropped image as a JPEG or PNG file (usually you don't have to worry about the file format or filename).
Your screenshot file is now ready.
The resulting file dimensions should be exactly 1280x720 pixels - on some systems of screens the file could be a lot larger (on Apple Retina displays for example). Don't worry about this, the file dimensions will be automatically adjusted once the file will be online.