I believe you open Instagram, go to the Home screen and click the circle w/ the “+” inside of it on the top left of the screen to record a new video/take a new photo for your Instagram Story.
To “collaborate” with another user, have the user send you their original content (photo or 10 second video), save that content to your phone and then re-open the Story screen. This time, however, swipe up to reveal photos/videos sent from within the last 24 hours that are available to upload. Click on the collaborator’s original content and then upload. Hope this helps! Let me know.