Once an object has been configured in the system, whether active or inactive, you are able to set up the various fields it can use.
From the main object list, click the object for which you want to set fields up. Past the object details, note the Fields header at the bottom of the form. Underneath the header will be a list of fields that are already configured for this object, as well as a Create New link which can be used to add an additional field.
For a description of the various properties of a field, see About Objects and Fields.
Click Save to save the field. Note that changes are not fully committed until you press the Save button from the main object list.
To edit additional attributes that do not appear in this form, find the Schedule Objects tab. Select the object that contains the field you need to change, and find the relevant field definition in the Fields related list.