Create your contact data field in the normal manner, giving it a unique name. Remember – your contacts will see the name of this contact data field, so give it some thought!
The default setting is private. Click on the radio button next to 'This data label is public' and click Save.
To change a contact data field that's already been created, go to Contacts > Contact data fields, click on the Default data fields or My data fields tab as appropriate, and then click on the Properties icon alongside the contact data field to edit its settings.