Basically, all you do is generate a public key pair and put the public key part (in ASCII) into a specially formatted TXT field in the database. Then, add the private key to your mail server. See:

https://www.mailhardener.com/kb/how-to-create-a-dkim-record-with-openssl

Use the following tool to validate your DKIM after you have generated and installed the keys

https://www.mailhardener.com/tools/dkim-validator

Generate DKIM keys https://www.mailhardener.com/kb/how-to-create-a-dkim-record-with-openssl Validate DKIM https://www.mailhardener.com/tools/dkim-validator