I am new to DotNetNuke and come from a WP/Drupal background. I can't find any documentation on what I feel should just be a simple thing to do.
I need to have three different layouts, a Homepage layout which just uses a simple 12 column gird.
An InteriorA page where it uses a 9 col grid with a 3 col
Then an InteriorB page where it uses a 3-3-6-12 grid.
I dont see where I build and save these templates in code, then where to assign them to the pages I want to use them on.
Following up on the previous answer, resources available to you include the Xcillion theme that is provided with DNN installs. You will find the entire skin in /Portals/_defauilt/skins/Xcillion.
The "page layouts" are the .ascx files. The details of the layout are the "content panes" included in the .ascx files. They are the divs with IDs and the runat-server attribute.
You can define the layouts using different skin files, and then put content (modules) in the content panes that you would like to use. In other words, you can include the 3 layout possibilities in one .ascx, and place content appropriately.
The DNN Wiki has http://www.dnnsoftware.com/wiki/dotnetnuke-skins, with covers the basics of theming. You should be able to find more resources by Googling. One great resource on theme construction is DNN Hero's tutorials from DNN Creating. Start at dnnhero.com. This is a subscription service, though.