If your registrar provides a web interface for managing the DNS records of your domain, then you can either use their web interface to fully manage your DNS, or you can use DomTool.
Before choosing either option, you will first want to request control of a domain using the HCoop Portal.
Be aware that it sometimes takes a while for the changes that you make to the DNS records to propagate to all ISP's, depending on how long various ISP's cache data. Usually by 3 days they are all updated.
Managing DNS using the registrar's website
If you want to use your registrar's web interface to manage DNS for you domain, you can do that. Just be sure to set the DNS option to noDns in the DomTool configuration file for your domain. Here's an example:
domain "yourdomain" where DNS = noDns with (* Other directives go here. *) end;
You should set up a CNAME (or an A) record for your domain pointing to shelob.hcoop.net, the current machine server users web sites.
Using Domtool to manage DNS
If you want to use DomTool to manage your DNS, then use your registrar's web interface to specify the nameservers ns1.hcoop.net and ns2.hcoop.net for your domain.
Then read the DomTool User Guide for instructions on using DomTool. There's also a thorough overview of the DNS directives at DomTool/Examples, in the DNS section.