You can find a complete set of instructions as well as links to files you'll need over on XDA Forums. For the file manager you could use ES File Explorer. This is one of the reasons I recommended it on Sunday as one of the 5 apps you should install on your KFHD.
But before you do anything, you should know that there's currently no way to fix your tablet if you mess up something while mucking about in the systems folder. This procedure is rather risky, so it might make sense to wait for a safer method. And I seriously doubt that the wait will be long; I learned last fall with the Nook Tablet and Kindle Fire launches that hackers will often find a better technique every few days.
So let's see. It's Tuesday, and the Kindle Fire HD first arrived on Friday. That means it took someone 5 days to root Amazon's new tablet, hack it, and install Google Play. Not bad, but other ereaders and tablets have had faster turnaround.
via XDA Forums