Yes, it did come with a guide, very easy to follow, though for the most part I did not need it.

You don't have to tap into the oem head unit. You can get your sound source at the subwoofer, by tapping into the wires that feed the factory sub. If you didn't have a factory sub you will need to go to the factory amp. They all should have one. There are quite a few articles on this site as to where to get the proper wires you will need. All you need for the install of the sub is included with the sub. For the amp you will need to buy an amp install kit.

If you are nervous still, I highly recommend paying an MECP certified installer a couple of bucks to put it in ($100.00) should cover box and amp if not less.