All the review process will take place in the online review forum. The first thing to ask yourself is if you already registered to Frontiers (note that being invited to review a manuscript will not create an account automatically).
If you have logged in and can’t find the manuscript in your account, please see "I cannot find the manuscript in my account".
If you have accessed the forum but don’t see how to download the manuscript, please see "I'm in the review forum, how can I download the manuscript".
If you have reviewed the manuscript, please see "How can I submit my review report?" about submitting your report and "I am a Reviewer. How can I send my corrections to the authors?" if you have to upload an annotated file as well.