Authenticated users must be able to flag webform submissions
Navigate to /admin/structure/flags/add/node and create a flag (do not check the "global" checkbox). For "Bundles" (on the same form, check "webform").

On that same form, review any of the various other options available to define/refine the flag to be created, such as:
- Flag Access,
- Display options,
- Link type,
- Etc.

Then use the "Save Flag" button.

A few other questions that may help to get you going (which are like variations of this question):


Any way to flag users who previously submitted a webform on revisit?
Adding a &quot;processed&quot; flag to a Drupal Webform, that admins can use to indicate whether or not they&#39;ve dealt with a particular submission?


Note: The Webform Flag Submissions sandbox is for D6 only (I'm assuming this question is about D7, is it?).