How do you create a new field?

I know how to add a field to a table, specifically VBAP for sales order item on additional data B tab.  what I am not really sure about is how to actually create the field I want to add to the table.

I understand all the naming conventions, etc.  ZZ whatever but I do not fully understand the steps to take and in what order or where to start.

Any help is appreciated.

Donna