First and foremost, you need an apple device - recent generation. For mac book you need one with touch bar if you don't have recent generation iPhone / iPad, I know it is hard to develop for Apple but nothing one can do about it :)
List of compatible devices can be found here.
Creating test account
Login with proper rights (to manage sandbox users) on Apple Developer site and go to People (available in account dashboard), go to app store connect from there. There is option to see all test users (last option in the menu in left side).
Add a user with proper region selected, make sure email is valid as you will need to verify this email before being able to use it. Verify the email and accept the terms.
Use the sandbox account
Once e-mail is verified, I recommend you to create a new user in mac (I don't think there is any option for this in iPhone / iPad) and login using this user in iCloud. Make sure you have a password in that user and have added a fingerprint.
Important: For this user, please make sure to select any region from the USA to be able to use the Wallet.
Once you have logged in with the sandbox user in iCloud, next step is to add test card in Wallet. Go to Wallet in Settings and add card, use the card from the list provided here. For integration with checkout.com use the cards from here.
That's it, now you can start paying on your test site.