Once you have accessed the site builder, click on the Try Sitejet button.

You can then select a template that you would like to use. 

If you prefer to customize a theme, select a Wireframe theme. This will give you full customization, starting from scratch.

Once you have selected a template, you can then begin customizing it. For example, to change a heading, you can double click on it and then replace the text.

To edit an element, you can click on it in the corner so that the menu comes up. From there, you can click on edit.