Issue new certificates in Keychain Access on Mac
As a CA, before you can issue a new certificate, anyone requesting a certificate must create a certificate-signing request (CSR) and send it to you via email. The CSR can be created using Certificate Assistant in Keychain Access. A CSR created by Certificate Assistant has the file extension .certSigningRequest.
In the Keychain Access app on your Mac, choose Keychain Access > Certificate Assistant > Open.
Select the CSR file you received.
To manually specify the information in the certificate, such as extensions and encryption, click “Let me override defaults,” then follow the instructions.
If you have questions while creating your certificate, click Learn More.