Once you access the Profile Card of the Workflow, either using the hyperlink from the notification e-mail, or with My InBasket or via the notification icon on the Dasboard, click on the "VOTE NOW" button (1) available in the "Signs Off" tab, then on "VOTE", and select "Approve" or "Reject" (2).

If you decide to reject, it is mandatory to add a comment.

Then confirm with the "COMPLETE" button (3).

Complete documentation: Design Verification Workflow FAQ