I had to do this once, and the way I went about it was to download a kickstart first and then work out what data I needed to put in based on that. So in other words, reverse engineering it. I think I started by doing Export Data on the Custom Data and then figuring out what to put in the different tabs based on what I know about what I pulled. In doing so, you might learn some new terms, like Parameter, Parameter Option, and Parameter Group.
This link contains some basic kick-start information. If you have never done a kick-start at all, it's worth it to step through the exercises even though none pertain to custom fields. The same things you learn importing a task into the system, e.g. what to put in the ID field, will be applicable for custom fields.
https://one.workfront.com/s/document-item?bundleId=the-new-workfront-experience&topicId=Content%2FAdministration_and_Setup%2FManage_Workfront%2FUsing_Kick-Starts%2F_kick-starts.htm&_LANG=enus
