To use the "Show Selected Country" feature you must know how to create a product in Cartfuel. Please read this article to learn how to create a product in Cartfuel.
Step 1: Show Selected Countries
When creating a new payment form in the product feature add-on area toggle on the "Add Shipping Fields" selector
A new toggle selector will appear titled "Show Specific Countries"
Toggle on the "Show Specific Countries" selector
A dropdown will appear. Select which countries you want to appear from the dropdown
Now any country that wasn't selected from the dropdown will not show up on your Carttfuel payment form