Convert your manuscript to a PDF file if you haven't done so already. In Mac OS X you can do this from the "Print" menu. In Windows you can usually do the conversion in the word processor of your choice, or you can use dedicated PDF conversion software.
Create an account at the self-publishing site of your choice.
Find the "Publish" button, or similar feature on the self-publishing site. It will often be on the main page of the site, but it could also be under an "ebook" or a "digitial media" subsection.
Follow the automated process of the site. The site will often have a wizard or setup utility to help guide you through the publishing interface.
Select a price and publish your book.