Issue new certificates with your own certificate authority
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 the Certificate Assistant in Keychain Access. A CSR created by the Certificate Assistant has the file extension .certSigningRequest.
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.