how to add/display something in dashboard/users/authors&users/edit user (wordpress)

Question

I want to add new user meta profile field for every wordpress user but only admin can edit it. I think I can use some code from

• Adding Custom User Meta to WordPress User Profiles | DQuinn.net
• Adding and using custom user profile fields

but I do not know

• where to place the code
• and how to not to edit core code of word press