Step 1: Login to the DNS settings panel of your domain provider.
Step 2: Create a new A record.
Step 3: As host fill in either @ if you want to just use your full domain (example dedicatedmc.io) or the sub domain name you want to you. Host ‘mc’ will let users connect to mc.dedicatedmc.io for example.
Step 4: As target fill in the IP of your server without the port.
Step 5: As TTL (Time To Live) select automatic if possible or select 30 minutes.
Step 6: Your players can now connect to your domain followed with the port. (example dedicatedmc.io:25566).
You can also get rid of the port behind the domain using by purchasing a dedicated IP and therefor be able to use the default Minecraft port or setup an SRV record which is a bit more complex and differs per domain provider per setup.