After clicking the edit button next to our chosen extra not in current use you'll see a page like this one:

Within that screenshot you'll see:

1. The admin note field. This is the label you would give an extra page. This label appears in column 2 of the previous screenshot and does not display anywhere on the client side.
2. This the url that would be used to get to this extra page on the client side. Note this is the "unrewritten url". To rewrite this url for SEO purposes see the link lower in this support wiki page
3. This is the page template attached to this extra page. This is the basic_page.tpl template by default. You can click the edit attachments link next to this to link to the page attachment admin tool page for this extra page.
4. This is the body template attached to this extra page. By default for "unused extra pages" this is set to the additional_page.tpl template. You can edit the body template attached to this extra page by clicking the edit attachments button

We already have a both body and page template attached to that page by default. But in this example we are going to use the default page template but assign our custom extra page body template to user for the body to display our custom content. To assign our brand_new_extra_page.tpl template for use on this extra page click the edit attachment button next to the current body template attached. By default that should be additional_page.tpl template. We chose extra page 15 for this tutorial and clicking edit attachments leads you to the page that allows you to attach templates to pages. You can alternatively get to this same "page to template attachment" page through the DESIGN > PAGE ATTACHMENT admin tool also.

Within the above screenshot you'll see:

1. the current template set this template assignment will be made within. This should be your custom template set and the same one started at the top of this process.
2. this is the direct url to this extra page on the client side
3. this is the page template assigned to this extra page. It is basic_page.tpl by default and we will not be changing this for the purposes of this tutorial
4. this is the body template that is assigned to this page. Here we'll select the extra page body template we created in the steps above.
5. once our custom body template is selected click the save button to save our attachment choice.

Now to test go to the url in 2 above to make sure the extra page appears as expected.