The first step in creating custom fields is to define the system codes that will give the custom data context. Before performing this step, you should first correctly identify what data type your custom data requirements are. See the Custom Data Overview for a discussion about this.
The System Codes screen is accessed by using the Administration > System Codes menu
Once the screen has been displayed, expand the category entitled "User Defined". This will display the 4 types of custom data types.
Creating the custom system codes is simply a matter of clicking on the appropriate system code type and clicking the New button, identical to the way regular system codes are administered. The only exception to his is when defining the name for a Drop Down Box where you first need to create the name for the box, then repeat the process for it's contents.
See also