This link goes to a detailed series of videos produced by NIH. If you would like more detail than what is given in this guide, these videos may help you.
The "W" questions answered
Who
Grant awardees as well as authors funded by NIH grants are responsible for policy compliance.
What
NIHMS is what you use to submit your article as an agreement of the NIH grant.
When
Immediately upon acceptance of your peer-reviewed article to a journal, it must be submitted it to PMC or NIHMS (which one can change depending on journal "method").
The policy has been in place since April, 2008.
Where
PubMed Central (PMC) is the repository where your article will be submitted.
Why
The public access policy was put into place because the United States government believes that results from government funded research should be available to everybody via the open web.
If you do not comply, NIH will stop non-competing continuation grants. They will also not fund future grant applications if you have a record of non-compliance.
Compliance Procedure Outline
Copyright : Own it. Read your agreement before submitting your article to a journal.
Journal "submission" Methods: A, B, C, or D. Which method does your journal use? see fabulous flow chart linked below