In the organizer panel, go to Data > Submissions. Select the submissions you wish to assign using the checkboxes.
Then click Assign reviewers.
Select to which reviewers you'd like to assign the submissions, then click Next.
Select the method:
Randomly distribute. This will randomly and evenly distribute submissions to the selected reviewers. Specify the number of reviews you expect for each submission.
Manual. This will assign all the selected submissions to all the selected reviewers.
Select if you want to notify reviewers by email. The email will indicate to the reviewer how many submissions were assigned and contain a link to access the reviews.