The National Institutes of Health (NIH) has put together a number of resources showing you how to use the NIH Manuscript Submission System (NIHMS). They include tutorials on how to submit your manuscript files, as well as instructions for signing into the system.

NIH Manuscript Submission System (NIHMS)
http://www.nihms.nih.gov/

FAQs: NIH Manuscript System (NIHMS)
http://www.nihms.nih.gov/faq.html

NIHMS System Slide Show
http://www.nihms.nih.gov/web-help/index.html