In order to save your stories for the day, first, you must tap on the story icon that is located at the top left corner of your feed, the one which bears your display picture along with the other story profiles of the users you follow. Upon tapping your story, you will find 3 dots situated at the top of your screen. Many choices will be shown if that is clicked. Tap MORE option then pick the SAVE option so that you can finally download your story. Render time will be required before you can see the video in your camera roll.