Ecommerce Transaction Tracking

Hi.

I am using Version 1.5.2.1 and I need to setup Google Analytics Ecommerce Tracking so that it tracks the transactions going through my site.

Is anyone able to tell me which files I need to edit, or know a link for a run-through of how I can do this..

I had it working in an old version before I upgraded and am now trying to get it to work again.

Many Thanks