NIH just released two new training videos that provide step-by-step guidance for PIs/Authors for two methods of submission and the approval process: 1) author self-submits the manuscript, and 2) publisher or third party submits the manuscript.  

Submitting an Article to PubMed Central (for PIs/Authors who self-submit)

Approving Submission of an Article to PubMed Central (for PIs/Authors who need to approve a submission made by a publisher or third party).

NIH Public Access Policy information.