In what follows, substitute your short username for myshortname
Create the file
containing the following text:
<Directory "/Users/myshortname/Sites/"> Options Indexes MultiViews AllowOverride None Order allow,deny Allow from all </Directory>Set the group and owner as follows:
sudo chown root:wheel /etc/apache2/users/myshortname.conf
Restart apache by switching Web Sharing off then on in System Preferences.