How to install a derailleur?

Question
I recently pulled a mt. bike out of the garbage.  Judging by the markings on it, it looks like it had been some sort of promotional bike that was given to a person who bought a Jeep Cherokee.  My guess is, the bike fell [hard] and the fall broke the derailleur.  Rather than fix it, the owner tossed it since the bike really isn't worth too much.  I would love to get it working, but I'm just a casual biker and don't want to put much money into it.  Point being, it is not worth it to me to take it to a bike shop and have a professional install and adjust a new derailleur.  I can buy a derailleur, but don't know how to put it on. (I've checked a few new derailleurs but they did not come with any instructions.)  Can you tell me how to properly install and adjust a derailleur (or, point me to somewhere that I can get instructions)?  Thanks.

Answer
Hello Curtis,

It is difficult to tell without knowing which derailleur you want to mount.
Every derailleur has there own instructions, which you can find on the most
websites of the most common brands/producers.
And otherwise I want to suggest to go to your local bikeshop, let him earn some money
on the bike. It will cost you a couple of bucks, and you don't get dirty hands as well.

Good luck

Mike