If you are using Google Workspace (Gmail for businesses) as Email Service Provider, this is the process to set up DKIM.
1. Log in to your Google Admin panel.
2. In the left hand side menu: Click on "Apps" then "Google Workspace" and finally "Gmail".
3. Generate a DKIM Key.
4. Go to your domain provider (GoDaddy, Cloudflare, Namecheap, Gandi, etc.) and sign in
5. Go to the page for updating your domain’s DNS records. It could be called "DNS Management", "Name Server Management", or "Advanced Settings"
6. Create a DNS TXT Record with the DKIM key generated in the previous step (Host Name, Record value and leave TTL value by default).
7. Go back to your Google Admin panel and click on "Start Authenticating" and wait for propagation (it can take up to 48 hours).
If you need additional information, you can directly refer to Google's documentation about DKIM set up: Set up DKIM for Google Workspace (Google Documentation)