You do it on the registrar's website. That domain is from Afraid though isn't it?
IIRC they didn't support NS records unless you contact them to have that support enabled. Check for an option to set up external name servers / NS on their site, but if you don't see it you'll need to ask them how to do that.
Their service has historically been known for not being the most compatible thing in the world due to its nonstandard nature...