Click on Listing then the Sales Channel you want and then click on Manage Listing Templates.
If you are creating a new template then click Create Listing Template or if you are editing an existing template then click the blue pen and paper icon under Actions.
Go through the template and make sure all the fields marked with * are filled out. Go through it until you get to the Optional Fields tab
You can now apply the Custom Fields to the template. So if for example on the product you gave it a different name and price change the Title and Start Price to the Custom Field name from the drop down.
Here you can also apply a HTML Template to the description if you want to know how to add a HTML template please click Here
Congratulations you have successfully added a Custom Field to a listing template. All you have to do now is save/ update the template