Using the Pro or Cornerstone editor, you can create a page then once it is done, you can save it as a template file.
Once you saved the page as a template, you could either go to the template manager and download the template file
If you download it, you will get a
.tco file. This will be useful if you want to use the same template to another site built with Cornerstone but is not necessary to do it if you will just use the template on the same site.
All you have to do is to create a new page in Cornerstone, then click on Load Template that is beside the save template option and you could choose which one of your existing templates to show.
Once it is loaded, you can freely edit the template element contents.
Hope this gets you started.