Importance of Peer Review 
Peer review is a critical element of scholarly publication, and one of the major cornerstones of the scientific process. Peer Review serves two key functions:
     Ensures research is properly verified before being published
     Rigorous review by other experts helps to hone key points and correct inadvertent errors

On Being Asked To Review
Are there any potential conflicts of interest?
A conflict of interest will not necessarily eliminate you from reviewing an article, but full disclosure to the editor will allow them to make an informed decision. For example; if you work in the same department or institute as one of the authors; if you have worked on a paper previously with an author; or you have a professional or financial connection to the article. These should all be listed when responding to the editor’s invitation for review. 

Conducting the Review (Double blind)
Reviewing needs to be conducted confidentially, the article you have been asked to review should not be disclosed to a third party. You should not attempt to contact the author. Be aware when you submit your review that any recommendations you make will contribute to the final decision made by the editor. The Critical comments on manuscript will be written in the following heads.

Overview of the reviw process

Recommendation to the Editor
Accept As Is
Requires Minor Corrections
Requires Major Revision
Reject the Manuscript (Please give reasons)
In the latter case, clearly identify what revision is required, and indicate to the editor whether or not you would be happy to review the revised article.