To create the subdomain:
- learn which registrar hosts your DNS records (you can use https://who.is/whois/100hires.com for that),
- sign in to your account on your registrar's website,
- choose your domain and go to its DNS settings,
- click “Add” to create the CNAME record,
- enter "careers" in the “name” field,
- enter "cname.hundredhires.com" in the "value" field,
- save it and then just ping us firstname.lastname@example.org to turn on the feature on our end.
Here are the instructions for GoDaddy https://www.godaddy.com/help/add-a-cname-record-19236 (pretty similar for any other registrars).