
Configuration
Creating a custom field for Contacts
Go to your Settings in the bottom left corner of your screen
Click Fields
Make sure you selected Contacts and click Add field:

Choose the field type
Enter the field name
Choose to make the field appear when adding a new Contact
Choose to make the field required
Choose to have the field as a filter option in your Contact page
Click Add field

Using Contact Fields
Filling in custom fields for a new Contact
Click Contacts, next click New Contact
Fill in the required fields (basic and custom fields)
Click Add (top of the screen)
Updating custom fields of an existing Contact
Click Contacts and select a Contact
Go to the Info tab
Modify one or more fields by clicking on it and typing or clearing its value
The Contact is automatically saved