You edit a file at
/etc/hosts. You should know how to perform basic editing operations in such Unix editors as
vim and
nano. The latter is easier and more intuitive.
When you enter the editor, put the lines from above so that they conform to the order of an
IP (dotted numeric sequences) followed by a
space followed by a
domain name (i.e.,
init.itunes.apple.com). Mind the bottom bar of the editor's window for the cues to the editing commands. Each of them should be enforced by pressing the
⏎Enter key. So, for example, "Write Out" is, if I recall correctly, the key shortcut
Ctrl-X Ctrl-W, then
Enter, then another one to exit (forgot the shortcut,
Ctrl-X perhaps). Done.