When you access a contact record in Redtail, you'll notice new menu items in your Left Menu Bar underneath Contacts (these menu items only appear when within a contact record):
Here we'll take a look at User Defined Fields (UDFs). After clicking UDFs from the menu, you will see any UDFs that have been assigned to the contact along with the field values for those UDFs (below is a partial sample from a contact record):
You can see here that this contact has multiple UDFs and values associated with his record.
If you want to edit the Value for a field, or to delete the UDF altogether from the record, you can click the Actions arrow to the left of the UDF and make the appropriate selection:
To add a new UDF and value for the contact, just click the Add button in the top right corner of this section.