Publishing from subdirectory

Hi,
I have old WordPress in my root directory and I have made new one in sub directory /WP. How do I publish the new site ?

I found this:
http://codex.ww.wp.xz.cn/Giving_WordPress_Its_Own_Directory

But do I edit the root directory’s .htacess by adding this in the end? or do I copy the subdirectorys .htacess and place that in root and add this on that, or how…

RewriteEngine On
RewriteCond %{HTTP_HOST} ^(www.)?YourDomain.com$
RewriteRule ^(/)?$ WP [L]

(do I add this in the end of other text in .htacess?)

… or is it better to copy the index.php and .htaccess and edit the index.php file. (what is the difference anyway?)

and then delete all the other “old WP “files.

What is the best way to proceed?