There is no "simple" way to explain how to design a site like those you linked to. It's a combination of graphics, html, CSS and understanding the Atahualpa theme. You should probably start with just playing around with the theme to get comfortable with what it does. There is a ton of documentation in the theme options. Then
w3schools.com is a good place to get some CSS info.
The sites you linked to use background images in the footer, sidebars, and body and telling you how to go about building them step by step is generally beyond the scope of this forum. If you start building the sites and have some specific areas that you get stuck on give a shout though. Maybe you could even contact the people who built the sites you linked to.
You could also look at the source of those sites to get an idea of what they did.