The code snippet should be placed just before the closing </body> tag in the source code of your site. Make sure to add it to all pages of your site in order to guarantee the best possible measurement. If you have a CMS site it can be easily done in the footer. See the example below:
<script type="text/javascript">
var __fs_conf = __fs_conf || [];
__fs_conf.push(['setAdv',{'id': 'xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxx'}]);
</script>
<script src="//analytics.freespee.com/js/external/fs.js" type="text/javascript"></script>