Custom domains are available on Professional and Business plans.
If you have trouble with any part of the process, be sure to contact us on firstname.lastname@example.org
Go to the DNS settings of http://yourwebsite.com and create a CNAME attribute pointing 'proposals' to 'your_subdomain.nusii.com.' (For lots of providers you have to add a dot after .com)
Here is an example of the private DNS settings for michaelkoper.com
'michael' is the subdomain in nusii and 'proposals' is my CNAME.
After you have this configured, drop us a line and we'll make sure everything is as it should be.