John, the .skin file is a standard asp.net file, not specific to Kentico at all. Essentially you add items in it with some "defaults" assigned to them.
So if your site's code name is "MySite", add a text file in the /App_Themes/MySite directory called Default.skin.
In the Default.skin file add the following:
<cms:CMSTextBox runat="server" AutoComplete="false" />
<asp:TextBox runat="server" AutoComplete="false" />
What this means is any instance of
asp:TextBox will by default have the
AutoComplete property set to false. If you added the
CssClass property with a value, it would add that to every instance of those controls as well.