Tell me how to use underscore.js as a template engine?

Everything you need to know about underscore template is here. Only 3 things to keep in mind:

<% %> - to execute some code
<%= %> - to print some value in template
<%- %> - to print some values with HTML escaped

That's all about it.

Simple example:

var foo = "blahblah";
var tpl = _.template("<h1>Heading for: <= fooo %></h1>");

this would be rendered in string <h1>Heading for: blahblah</h1>