To add a fixed-position template, you'll need to first create a container on your page for an inline offer or template (Piano supports all selectors supported by JQuery). You'll then need to nest that container within another container, which you will position on your page using the CSS position property. Here's what a template/offer container called piano_inline would look like nested within a wrapper container called piano_wrapper:
<div id="piano_wrapper">\n <div id="piano_inline"></div>\n</div>
You would then modify the CSS of your page to position piano_wrapper at the bottom of the page (or wherever else you might want it placed). Here's what that CSS might look like:
#piano_wrapper {\n position: fixed;\n bottom: 0px;\n width: 100%;\n z-index: 999999;\n left: 0;\n}
You would then add #piano_inline to the "Selector for Inline Container" field in the Show Offer or Show Template card. The template/offer will be invisible until it's triggered based on the rules you set in Composer.