After looking through the mentioned tutorial on the default extra pages above you should now know that each page is made up of two templates. The page template which will include the default header and footer and the body template which will include your custom content. The page template already exists but your custom body template does not. So to create that template lets create a blank template within the system where the other extra page templates are. You can do that easily through the following admin tool:

DESIGN > MANAGER

Within the above screen you would:

1. navigate to DESIGN > MANAGER admin tool
2. make sure you are editing your custom template set
3. click the /main_page/extra_pages/ directory

Now you should be looking at all the default extra page templates that come with the software by default. You will next click the create new file link within that admin tool as shown in the following screenshot:

That will make the following popup appear:

Within that screenshot you would insert the name of the new extra page body template. We have inserted brand_new_extra_page and clicked enter. That will create a new template in the /main_page/extra_pages/ directory as shown in the screenshot below.

Now you see the brand_new_extra_page.tpl template within the other extra page templates of the /main_page/extra_pages/ directory. Now to edit that template to put your custom content click the checkbox next to that template and then the View/Edit button at the bottom of that admin tool as shown above.

Once you click the View/Edit button you'll now see the DESIGN > MANAGER open that page for editing. We just inserted something into that template for illustration purposes as you see below in with that template open for editing:

Once you are done inserting your content click the Save Changes button at the bottom of that editor box.

Now you have custom extra page body template with custom content within it. Now to find an extra page within the system to attach it to.