Demanding Boss

How would you handle a boss that does not allow you to focus/work on one task at a time? I’m finding it very difficult to understand how I can plan my work day when I am constantly be interrupted and told to stop what I’m doing and start something else. My boss does not manage her time well and it trickles down and affects my time and my work load (work for clients). Ultimately, some tasks don’t get completed on time due to being pulled from that job. How should I deal with this?
Thank you.