What advice are you looking for? How to position it so it is correctly behind the elements it is supposed to be the background of?
A lot of "it depends" there.
Ideally, you would do something like:
And make the height and width of all 3 divs identical.
<div style="position: relative;">
<div style="position: absolute; z-index: 1;">...background contents...</div>
<div style="position: absolute: z-index: 2;">...foreground contents...</div>
Alternatively, you can position the background normally and then use absolute positioning (as just above) to place the stuff that goes in front of it. (Which is what I usually do, esp. for slideshows, but your mileage may vary.)
var node = document.getElementById("someExistingContent"); // or however you find it
var x = 0;
var y = 0;
while ( node != null )
x += node.offsetLeft;
y += node.offsetTop
node = node.offsetParent;
var bg = document.getElementById("someDivWithPositionAbsolute");
bg.style.top = y + "px";
bg.style.left = x + "px";