For DKIM, first you will want to get the DKIM record from the Communications->Settings->Domains page in your Boathouse Connect account to add to your DNS. After you have added a new domain, click the red "Verify" buttons to view the DKIM / Return-Path Record details as well as verify if you have properly added them to your DNS.
DKIM records are added to your DNS as a TXT record and Return-Path records are added as CNAME records. Usually your DNS is managed by your hosting provider, so you would want to head there to add these records. This is not always the case, however. If you are using a DNS provider, such as DNSimple, you would need to add your DKIM and Return-Path records there instead of with your hosting provider.
Once you have the DKIM and/or return-path records from your Boathouse Connect account, below are some links to instructions on adding DNS records for common hosts and DNS providers.
Boathouse Connect