Converting HTML site to WordPress.org

Apologies if this has been dealt with before; I just can’t find it.

I am currently using Hostmonster’s CPanel with cpanel to install wordpress.com. My existing site is html and I want to replace it with a site using wp.org. I have read the “how to move” article but I am not 100% certain on how get the wp.org files on my existing website’s install of wp.com.

I’m assuming I download the .org files then upload them to a folder within my existing site. When the new site is ready, do I move this to the public_html directory and move the old site to another folder? Do I even need to download the WP/simple scripts version of wp.com? I just don’t want to lose the existing site.