First, create your subdomain and make sure your subdomain is pointed to Yola's IP address: 184.72.229.176.
Then, publish your site to a free .yolasite.com subdomain.
Once you have done this, please get in touch with the Customer Service Team and let us know what your yolasite.com subdomain is and what custom subdomain you would like assigned to your site. One of our technical team members will be happy to do this for you.