How do I get all imported items to use my WP-Lister template?

Assuming that you already have created a listing template and that all imported products are still using the default listing profile, all you have to do is to edit that profile and select your listing template.

Then visit the listings page and use the Preview option to see how the eBay listings would look - if they were unlocked and revised on eBay. (Since all your imported listings are still locked, only price and quantity changes are updated on eBay.)

To update the eBay listing with all the data from WooCommerce and WP-Lister (listing profile and template), you first need to unlock your items - preferably a few test items at first - and revise them. Check on eBay is the result looks all right, before you visit the tools page where you can unlock (or lock) all listings with a single click. From there it's just one more click on "Revise all changed items".

See  About locked vs unlocked listings for more details.