<p>What is the simplest way to install memcached on CentOS for someone new to the world of Linux?  What is the best way to enable it for Apache and PHP</p>

Unless Apache and PHP have some option to utilize memcached for internal workings (of which I am unaware of), you typically don't "enable" it for such apps.  Instead, you would get a
client library
to use memcached from within your application, then start up memcached on whatever servers you want to provide memory with, then just use the client library API to store and retrieve cached data across multiple servers.