As you do with any other class - assign it to the Zend_Registy. I'd suggest setting like this:
Zend_Registry::set('Zend_Log',$logInstance);
This is a common way, which is used also for translate (set translate instance to 'Zend_Translate' and classes like forms and validators will find it automatically).
You can use Zend_Registry::get('Zend_Log')->log(...)
to log anywhere you want. It's not very good from the point of architecture (you should not use normally), but for log - which can appear practically anywhere in the app from view helpers to controllers and models it's a good thing.