Log in to your shop admin

Visit app shop
https://apps.shopify.com/cardinity_external_payment 

Click Add App

If you have multiple stores under the account it will ask for a store select.
After selected the store it will redirect to the authorization page.

Click Install App
It will take you to setup page. Fill up your cardinity credentials. (you can find them on https://my.cardinity.com/integration/api )

Click Save

On the next page Activate Cardinity Online Payments to use it on checkout.

Now you can use Cardinity Online Payments on checkout.