1. Please go to your account and click Settings menu on the left under Shop Section.
2. For sub domain setup, click on the Domain tab, enter an available subdomain name. e.g. demo, then your website address will be demo.just.co.nz
2. For domain setup, enter an your own domain name. e.g. abc.co.nz, then your website address will be www.abc.co.nz
You need config your domain DNS record where you bought the domain.
1) Add "A" record to 126.96.36.199 for abc.co.nz
2) Add "CNAME" record to justcar.co.nz for www.abc.co.nz
4. For Custom UI setup, click on the Custom UI tab.