Step 1: To edit a field, simply click the green wrench icon to the right of the desired field. A pop-up menu (shown above) will appear, where you can edit all aspects of the field.
Step 2: You can make the desired field public or not by using the checkbox labeled "Public".
Step 3: Making the field required means the field must be filled out. To make a field required or not, simply use the checkbox labeled "Required" at the bottom of the pop-up menu.