You can use DKIM wizards to generate your DKIM public and private key pair. There are a lot of DKIM wizards, however, Port 25 is a good option.

Follow these steps to generate DKIM keys using Port 25’s site:

1. Use Port 25 to enter the From Domain that you are authenticating.
2. Enter the selector name. Choose a selector name that describes the type of email you are sending, such as marketing or newsletter. You should also standardize your selector names for ease of tracking.
3. Choose 1024 bits or 2048 bits for your desired key size. Due to DKIM specification, make sure your key is at least 1024 bits.
4. Select Create Keys.

Here is a link to the Port 25 tool mentioned in this article: Port 25