First, you must create a new mailing list before you can create custom fields. Once you have done that, proceed to the following steps.

1. Go to Contact List > Create Custom Fields.
   
   
2. Choose field type (Text Field, Multi-Line, Number, etc.), name it and choose whether you want that field required for future sign-ups. Click next.
   *Note that if you'd like a Phone Number field, use Text Field as the Number field will not accept dashes and will error out.
   
   
3. Enter any default values and field length limits. The key here is you want First Name and the list you import has subscribers with no first name. Think of what you'd want merged in as a default ('Friend', etc).
   
   
4. Then, choose what lists you want that field attached onto.
   
   
5. Hit 'Save' and you're all set. Simply follow the same procedure for the rest of the fields and you're good to go!