I have a WP installed on a subdomain blog.domain.com. What do I have to do in order to get it moved into domain.com/blog directory?
is the folder for the subdomain named the same as the folder you want to run from? If so, it may be as simple as getting rid of the subdomain.... no longer pointing it to the folder. And then changing the 2 urls in settings-> general.
Otherwise, review the moving instructions in the codex if you need to physically relocate the files http://codex.wordpress.org/Moving_WordPress