how do i make a small overlay at the bottom right corner with z-index 5?

i need a small overlay to display at the bottom right corner of the page

i can't get the position right

div#overlay {
z-index: 5;
float: right;
}

div#overlay { position:absolute; bottom:0; right:0; }

Something like this? z-index is useless without the element having a position of absolute/fixed/relative.