Click Add/Edit Registration Custom Fields on the Events->Guest List tab in the Guest List Summary area.
Field Name is the internal name of the field. It shows up on the guest list download.
Public Display Text is how you want it presented on the registration screen. HTML is allowed, e.g. to create links.
Required is for ensuring the registrant supplies data
View the data by clicking into each registration or by downloading the guest list under the Reports drop-down menu at the bottom of the Guest List.