To manually generate a Certificate, you need a Certificate Signing Request (CSR) file from your Mac. To create a CSR file, follow the instructions below to create one using Keychain Access.
Create a CSR file. In the Applications folder on your Mac, open the Utilities folder and launch Keychain Access.
Within the Keychain Access drop down menu, select Keychain Access > Certificate Assistant > Request a Certificate from a Certificate Authority.
In the Certificate Information window, enter the following information: In the User Email Address field, enter your email address. In the Common Name field, create a name for your private key (e.g., John Doe Dev Key). The CA Email Address field should be left empty. In the "Request is" group, select the "Saved to disk" option. Click Continue within Keychain Access to complete the CSR generating process.