You would be putting the code for this in your Theme page. You can get there by Hovering over My Gaia, then clicking on Account. Youll then see the Theme link on the right side, so click that and you put the code in the box there. Also on the Account page, in Preferences make sure your layout is set to current and not classic (Right under the timezone) when you use these layouts otherwise the layout wont look as expected.
We do offer a GaiaOnline Layout Generator for people to use.