First ensure that you open the proper document software for the document you are trying to view. Trying to open a sketch document in flow will result in an error.
Once the software is open go to the main menu (hamburger icon) and select 'Open'. In open find the option that says 'Open Shared'. In the Open Shared menu there is a section to input the document ID. Once you are done hit enter. Please note that you may need to enter a password if the document is password protected.