301 Redirect Help

I am not sure if I have this in the right spot, but here goes.

I had an issues with a word press installation. So I ended up re installing a fresh copy and loading my database to it. Plus I changed the directory it was in.

So now I have 2 directories with duplicate content. So I need to delete the old one and do a 301 redirect, but here is where I am lost.

mysitedotcom/old_directory/category/post_name/

mysitedotcom/new_directory/category/post_name/

are the same.

So How would I add a file, or modify the .httaccess where if someone clicked a link that was old it would take them to the new link, so I can delete the old installation.

For example there is an old link mysitedotcom/old_directory/...ake-cool-beans

and some one clicks it I want them to be routed to

mysitedotcom/new_directory/...ake-cool-beans

Any help would be appreciated.