The default reviewer will be the workflow owner
Click on the pencil to change the default reviewer and/or add more reviewers. You can add as many reviewers as you like. Once one of them acts on the request, it is considered done.
Hit Enter or Return on your keyboard after each email entry.
Once you have added all your reviewers, click Update.
If you want to add Conditional validation reviewers, click the + button (1) to enter the conditional validation criteria (2) and Reviewer(s). You can add multiple Conditional validation & Reviewers for each validation step
Our blog post here: http://goo.gl/UUh67f explains the process in details
Repeat to add as many validation steps and reviewers as you need