Hi!
When you are creating a listing, go down to the section where you can select a template. Open the template gallery section by clicking "Select Template" or "Choose Different Template". On the left-hand side of the window that opens you'll see categories of templates. Select "Custom" and any templates that you've customized will appear that you can choose from. Hit "Select" for the template you want to use, and on the next screen be sure to confirm your selection by clicking the "Save Template to Listing" button on the left. Your custom template will now be applied to your new listing that you are creating.
Hope that helps!