I will help you get started, but there is a lot of reading you need to do.
I assume that your Xbox 360 is already modified? If not, this is your first step. You either need to find a guide to modify your Xbox 360 yourself, or have somebody else do it. I assume that if you were capable of doing it yourself, you wouldn't need this guidance, so you will probably want somebody else to do it. The going rate is about $40 in the US. If you do want to do it yourself, you need to find a guide. The most recent version of the firmware that you will flash to your Xbox 360's DVD drive is called iXtreme 1.6. The software you use to update your firmware is called Jungle Flasher. I don't think I can include links, so let's just say that torrents are your friends.
If your Xbox 360 is already modified, then things are much simpler. You need two programs. The first of these programs checks your downloaded ISO file to make sure that it is 100% legit and as safe as possible to run on Xbox 360. This program is called ABGX360 and there are guides for this program as well. This step is extremely important.
After you figure out that your ISO file is safe, you need to get ImgBurn. This program is a freeware DVD burning program. You can find it on Google.
To use this program to burn your ISO you will need blank DVDs that are DUAL LAYER, meaning they hold more informatioon (7.8GB?) than standard SINGLE LAYER DVDs which only hold something like 4.8GB. All Xbox 360 games will need the Dual Layer, or DL, DVD. Also, make sure you get a good brand, as there is a big difference here. The brand you need is Verbatim. I've also had luck with Sony, but Verbatim is the accepted standard.
If this has all gone over your head, I highly recommend you get your work done by a pro and just pay for it. It's still a good deal compared to $60US a game.