Hi,
To use the Auctiva templates, you just need an Auctiva account that's link (by token) to your eBay account and use of the Auctiva one-page lister to create listings. You select a template for use on the one-page lister, which is very similar to eBay's one-page lister.
If you don't want to list from Auctiva, you can use the HTML icon at the right of a listing on the Saved Listing page to copy and paste the code to eBay's one-page lister.
Danno