Click on "Edit HTML" in Blogger and then insert the following in the <head> section of your HTML:
<script src="http://alexgorbatchev.com/pub/sh/2.1.364/scripts/shCore.js" type="text/javascript"></script> <script src="http://alexgorbatchev.com/pub/sh/2.1.364/scripts/shBrushRuby.js" type="text/javascript"></script> <script src="http://alexgorbatchev.com/pub/sh/2.1.364/scripts/shLegacy.js" type="text/javascript"></script> <link href="http://alexgorbatchev.com/pub/sh/2.1.364/styles/shCore.css" rel="stylesheet" type="text/css"></link> <link href="http://alexgorbatchev.com/pub/sh/2.1.364/styles/shThemeDefault.css" rel="stylesheet" type="text/css"></link> <script language="javascript"> window.onload = function () { dp.SyntaxHighlighter.ClipboardSwf = 'http://alexgorbatchev.com/pub/sh/2.1.364/scripts/clipboard.swf'; dp.SyntaxHighlighter.HighlightAll('code'); dp.SyntaxHighlighter.BloggerMode(); } </script>
and the following into the <body> wherever you want to put your code:
<pre class="brush: ruby" name="code"># Your Ruby Code</pre>