Create a CSV/Excel file where each row is a contact. The column headings can be anything you want. You can map them in the next step.
A valid example using two default fields (cell and firstname) and two custom fields (c1 and c2):
cell , firstname, c1 , c2
27112345678, mike , 12000, Gauteng
27112345679, sarah , 23000, Western Cape
Download Example CSV