Before you install your 2 Column Background Layout First make sure your original installed template is Minima.
To do this, do the following:
At the very top, right hand side of the blog - Click Customize.Click the Layout Tab.Click the Edit Html Tab.In your html file, Look for the type of template you have. It shouldsay something like this..
Blogger Template Style Name: Minima
If you do not have a Minima template, select a new template style by clicking the Layout Tab and Select New Template Tab.
Changing your template should not delete any of your gadgets or text.