[checkoutemail] make more fields "required"

Hi I use checkout and checkoutemail modules.
I have searched trough the forum and found these threads:

http://gallery.menalto.com/node/76858
http://gallery.menalto.com/node/91725

However I can't seem to find the thread that should have "long-winded answer"?

As I understand I need to modify Details.inc to include "if statements" to check if the field is not empty.

Understand me right I need to convert the already present fields to required, for example
first name
last name