Also check the page where you found the patch carefully. Sometimes you can find an updated ebuild that works with that patch in the same page, so you don't have to patch it yourself.
You need to examine the patch and clear up whether it's a patch for the app or for the ebuild itself. If it's a patch for the ebuild you just need to copy the ebuild into an overlay and patch it. But if it's a patch for the app you need to modify the ebuild so it applies the patch to the app after unpacking it and before compiling. This is the official documentation about how to correctly alter the portage tree so you don't mess up anything. Either way you need to create an overlay. That's for sure. http://www.gentoo.org/doc/en/handbook/handbook-x86.xml?part=3&chap=5 -- Jesús Guerrero
