This works best on a Minima background - so the first thing you need to do is save your template before you do anything (then if it all goes wrong, you won't lose anything).
If you want a 3-column background, then Minima is the best one to choose (it's the easiest to adapt).
Back up your blog before you do anything if you are new at this, and of a nervous disposition!
Go Layouts > Add a Gadget at the bottom of your blog - this actually helps the blog to load faster (and you aren't likely to delete the box by mistake when adding buttons etc).
Click HTML/JavaCode box - this is where you are going to add the code for the background template.
Copy&paste the code into this box, Save - and that's it! Go and admire.
If you don't like it, just click to open the box again, and press Remove.