Please navigate to Data Management > Custom Fields
Once in the "Custom Fields" page, locate the "List ID" you are looking to add the custom fields on, and under "Actions" click on the + icon to open the fields page
Once on the page below, click on "Add custom field"
Once on the form page below, please fill out the necessary fields and click on "Save"
The field will now have been added to that list.
Note: Ytel recommends that custom fields be used only if the default fields are not sufficient for your leads. In our experience, not using any of the default fields and just using custom ones can make things much harder to keep organized