The following FAQs link to slide shows that demonstrate step-by-step procedures illustrated with screen captures of the NIHMS website.
The NIHMS website has additional FAQs.
There are four approaches to submitting articles to PubMed Central (PMC): Methods A, B, C, and D.
The method selected depends upon the degree to which the article journal/publisher participates in the process. The NIH Public Access submissions methods page provides an overview of how these methods differ, information about the journals and publishers that participate in each method, and step-by-step instructions.
The NIH Manuscript Submission System (NIHMS) is required for submission methods C and D.