After you sign in to the website by going to the Reviewer Area, If there is any new invitation it will be specified like this:

By clicking on the new invitation you can reach to the page bellow which you can Accept the invitation or decline the invitation.

If you accept the invitation according to the deadline you have to complete the task. The deadline is accessible through:

For accepting the invitation click on the accept invitation button Like this:

A new page will be open and ask for your comments:

By clicking on the submit comments a new page will be open which you can write 2 different kinds of comments:

The first comment will be visible for Associate editor and the Author

This one is a kind of private message between the Associate editor and the Reviewer.

By clicking on the submit button the task will be completed.

In the situation you want to decline the invitation click on the decline invitation button

A new page will be open and ask you to submit the reason you want to decline. By click on the submit button the process will be ended.