To create a certificate for your Merchant ID, perform the following actions.
In the personal account of Apple Member Center, go to Certificates, Identifiers & Profiles.
On the page that opens in the Identifiers section on the left, select Merchant IDs.
Select your Merchant ID from the list and click Edit.
Click on Create Certificate in block Apple Pay Payment Processing Certificate, then click Continue.
Click Choose File and specify the path to the file with the certificate signature request downloaded from the personal account of the payment gateway.
Click Generate.
Click Download, to download the created certificate to your computer.
After the certificate has been downloaded, click Done.
If you have completed the above mentioned actions, you can start developing the interaction of your mobile app or site with the payment gateway.