Not sure what you are trying to do. Do you mean you want to "Open With" Automator? Automator is a program for creating repeatable scripts. You wouldn't open files with it, unless they were automator scripts.
Do you want to create an automator workflow and then run that workflow against some files? If so, then it is a matter of creating the workflow in Automator, exporting it, and then running it (if it is the type of workflow where it asks for files).
Not sure what you are trying to do. Do you mean you want to "Open With" Automator? Automator is a program for creating repeatable scripts. You wouldn't open files with it, unless they were automator scripts.
Do you want to create an automator workflow and then run that workflow against some files? If so, then it is a matter of creating the workflow in Automator, exporting it, and then running it (if it is the type of workflow where it asks for files).