Go to People Search for the reviewer you will be assigning submissions to
Next to the appropriate name, hover over the arrow in the 'Action' column and click 'Assign to Submissions'
Search by Completed = Yes and any other search criteria (e.g., Category)
Check the box next to the proposals the reviewer should be assigned to and click 'Assign'
NOTE: you may also use the 'Check/Uncheck All' option to quickly assign a reviewer to all submissions on the page